Unit 4 (Week 13 Patterns of Inheritance) Flashcards
What is the transmission of characteristics from parent to offspring?
Inheritance
What was the idea that the determinants of hereditary traits are transmitted in discrete units, or particles, from one generation to the next?
Particular inheritance
[Information on Different Types of Mendelian Inheritance Patterns and Their Molecular Basis]
Simple Mendelian inheritance
Inheritance pattern: Pattern of traits is determined by a pair of alleles that display a dominant/recessive relationship and are located on an autosome. The presence of the dominant allele masks the presence of the recessive allele.
Molecular basis: In many cases, the recessive allele is nonfunctional. Though a heterozygote may produce 50% of the functional protein compared with a dominant homozygote, this is sufficient to produce the dominant trait.
X-linked inheritance
Inheritance pattern: Pattern of traits is determined by genes that display a dominant/recessive relationship and are located on the X chromosome. In mammals and fruit flies, males are hemizygous for X-linked genes. In these species, X-linked recessive traits occur more frequently in males than in females.
Molecular basis: In a female with one recessive X-linked allele (a heterozygote), the protein encoded by the dominant allele is sufficient to produce the dominant trait. A male with a recessive X-linked allele does not have a dominant allele and does not make any of the functional protein.
Incomplete dominance
Inheritance pattern: Pattern that occurs when the heterozygote has a phenotype intermediate to the phenotypes of the homozygotes, as when a cross between red-flowered and white-flowered plants produces pink-flowered offspring.
Molecular basis: Fifty percent of the protein encoded by the functional (wild-type) allele results in an intermediate phenotype.
Codominance
Inheritance pattern: Pattern that occurs when the heterozygote expresses both alleles simultaneously. For example, a human carrying the A and B alleles for the ABO antigens of red blood cells produces both the A and the B antigens (has an AB blood type).
Molecular basis: The codominant alleles encode proteins that function somewhat differently from each other. In a heterozygote, the function of each protein affects the phenotype uniquely.
Epistasis
Inheritance pattern: A type of gene interaction in which the alleles of one gene mask the effects of an allele of another gene.
Molecular basis: Two different genes are needed to produce a given phenotype. Loss of function of one of the genes alters the phenotype.
Continuous variation
Inheritance pattern: A pattern in which the offspring display a continuous range of phenotypes.
Molecular basis: This pattern is produced by the additive interactions of several genes, along with environmental influences.
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[Start 17.1 Mendel’s Laws of Inheritance]
In 1856, Gregor Mendel began his historic studies on what plant?
How long did he analyze this plant?
What was the name of his published paper in 1866?
Why was his paper ignored by peers?
Pea plants.
8 years!
“Experiments on Plant Hybrids”
His paper was largely ignored because of the title.
What is the process in which two individuals of the same species with different characteristics are bred or crossed to each other; the offspring are referred to as hybrids?
Hybridization
What was Mendel particularly intrigued by?
The consistency with which offspring of such crosses (different characteristics of plants in the same species) showed characteristics of one or the other parent in successive generations.
How did Mendel, with a foundation in physics and natural sciences, lead him to uncover the naturals laws governing inheritance?
He carried out quantitative experiments in which he carefully analyzed the numbers of offspring carrying specific traits.
What is a characteristic of an organism, such as the appearance of seeds, pods, flowers, or stems in the garden pea?
Called a character.
Why did he choose this species? Several properties of the garden pea were particularly advantageous for studying inheritance. First, it was available in many varieties that differed in characteristics.
What is an identifiable characteristic; usually refers to a variant?
Trait
What was another advantage of studying the pea plant?
They are normally self-fertilizing.
What is fertilization that involves the union of a female gamete and male gamete from the same individual?
Self-fertilization
What is a strain that continues to exhibit the same trait after several generations of self-fertilization or inbreeding?
True-breeding line
What was the third reason why pea plants were used in hybridization experiments?
Ease of making crosses: The flowers are fairly large and easy to manipulate. In some cases, Mendel wanted his pea plants to self-fertilize, but in others, he wanted to cross plants that differed with respect to some character, a process called hybridization, or cross-fertilization.
What is cross fertilization?
Fertilization that involves the union of a female gamete and a male gamete from different individuals.
Why are the stamens removed from the purple flower in this cross-fertilization procedure?
The stamens are removed from the purple flower to prevent self-fertilization.
What is a cross in which an experimenter follows the variants of only one character?
Single-factor cross
As an example, we will consider a single-factor cross in which Mendel followed the tall and dwarf variants for height
What does P generation mean?
The parental generation in a genetic cross.
What does F1 generation mean?
The first generation of offspring in a genetic cross.
What is the The F1 offspring, also called single-trait hybrids, of true-breeding parents that differ with regard to a single character?
A monohybrid.
Why do offspring of the F1 generation exhibit only one variant of each character?
The reason why offspring of the F1 generation exhibit only one variant of each character is because one trait is dominant over the other.
How is the second generation of offspring in a genetic cross denoted?
F2 generation
What was the three important ideas that Mendel postulated about the properties of traits and their transmission from parents to offspring?
- Traits may exist in two forms, dominant and recessive.
- An individual carries two genes for a given character, and genes have variant forms (now called alleles).
- The two alleles of a gene separate during the process that gives rise to haploid cells and gametes, so each sperm and egg receives only one allele.
What are the alternative traits described as?
Dominant and recessive
What refers to the trait that is displayed in a heterozygote?
Dominant
What refers to a trait that is masked by the presence of a dominant trait in a heterozygote?
Recessive
Tall stems and purple flowers are examples of dominant traits; dwarf stems and white flowers are examples of recessive traits. In this case, we say that tall is dominant over dwarf, and purple is dominant over white.
What is a unit of heredity?
A gene. At the molecular level, a gene is an organized unit of base sequences in a DNA strand that can transcribed into RNA and ultimately results in the formation of a functional product.
What coined the word gene?
A term coined by the Danish botanist Wilhelm Johannsen in 1909. Comes from the Greek word, genos, meaning birth.
What did Mendel postulate about genes?
Mendel postulated that every individual carries two genes for a given character and that the gene for each character in his pea plants exists in two variant forms, which we now call alleles.
What did Mendel always observe via a ratio in dominant and recessive traits?
3:1 ratio.
How did Mendel interpret this ratio? He concluded that each F1 plant carried two versions (alleles) of a gene affecting height (or another character) and that the two alleles carried by such an F1 plant separate, or segregate, from each other during the process that gives rise to gametes.
Therefore, each sperm or egg carried only one allele.
Remember: Tt combinations, or dominant-recessive combination of gametes, can occur in two different combinations. This means the dominant trait can come from the egg or the sperm.
This also means that Tt offspring are produced twice as often as TT or tt offspring.
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Which law states: The two alleles of a gene separate (segregate) from each other during the process that gives rise to gametes, so every gamete receives only one allele?
Mendel’s law of segregation
What is the ratio of the T allele to the t allele in the F2 generation? Does this ratio differ from the 3:1 phenotype ratio? If so, explain why.
The ratio of alleles (T to t) is 1:1. The reason why the phenotypic ratio is 3:1 is because T is dominant to t.
What is the genetic composition of an individual?
The genotype.
[Use Picture Mendel’s Analysis] TT and tt are the genotypes of the P generation, and Tt is the genotype of the F1 generation. In the P generation, both parents are true-breeding plants, which means that each has identical copies of the allele of the gene for height.
What do you call an individual with two identical copies of an allele?
What about an individual with two different alleles of the same gene?
Homozygous
In the specific cross we are considering, the tall plant (TT) is homozygous for T, and the dwarf plant (tt) is homozygous for t.
Heterozygous
Plants of the F1 generation are heterozygous, with the genotype Tt, because every individual carries one copy of the tall allele (T) and one copy of the dwarf allele (t). The F2 generation includes both homozygous individuals (homozygotes) and heterozygous individuals (heterozygotes).
What is the characteristics of an organism that are the result of the expression of its genes?
Phenotype.
Although the F1 offspring are heterozygous (Tt), they are phenotypically tall because each of them has a copy of the dominant tall allele. In contrast, the F2 plants display both phenotypes in a ratio of 3:1.
What is a common method for predicting the outcome of simple genetic crosses?
What are the steps to do a Punnett square? (5)
A Punnett square, developed by British geneticist Reginald Punnett
- Write down the genotypes of both parents. In this example, a heterozygous tall plant is crossed to another heterozygous tall plant. The plant providing the pollen is considered the male parent and the plant providing the eggs, the female parent. (In self-pollination, a single individual produces both types of gametes.)
Male parent: Tt
Female parent: Tt
- Write down the possible gametes that each parent can make. Remember the law of segregation tells us that a gamete contains only one copy of each allele.
Male gametes: T or t
Female gametes: T or t
- Create an empty Punnett square. The number of columns equals the number of male gametes, and the number of rows equals the number of female gametes. Our example has two rows and two columns. Place the male gametes across the top of the Punnett square and the female gametes along the side. (Outside the box)
Step 4. Fill in the possible genotypes of the offspring by combining the alleles of the gametes in the empty boxes.
Step 5. Determine the relative proportions of genotypes and phenotypes of the offspring. The genotypes are obtained directly from the Punnett square. In this example, the genotype ratio is 1 TT : 2 Tt : 1 tt. To determine the phenotypes, you must know which allele is dominant. For plant height, T (tall) is dominant to t (dwarf). The genotypes TT and Tt are tall, whereas the genotype tt is dwarf. Therefore, our Punnett square shows us that the phenotype ratio is expected to be 3 tall : 1 dwarf.
- What can we conclude if an individual has a recessive phenotype?
- What about if an individual has a dominant phenotype?
- It is homozygous for the recessive allele.
In this example, a plant that is dwarfed in height, has a genotype of tt which means it is homozygous for the recessive allele.
- An individual with a dominant phenotype may be either homozygous or heterozygous—a tall pea plant may have the genotype TT or Tt.
How do we distinguish between the two possibilities of a individual’s genotype (either having all dominant, TT alleles or heterozygous allele Tt) who has a dominant phenotype?
Mendel devised a method called a testcross to address this question. In a testcross, the researcher crosses the individual of interest to a homozygous recessive individual and observes the phenotypes of the offspring.
Essentially, a cross to determine if an individual with a dominant phenotype is a homozygote or a heterozygote.
Let’s suppose you had a pea plant with purple flowers and unknown genotype and conducted a testcross to determine its genotype. You obtained 41 offspring plants: 20 with white flowers and 21 with purple flowers. What was the genotype of the original purple-flowered plant?
The genotype was Pp. To produce white offspring, which are pp, the original plant had to have at least one copy of the p allele. Because it had purple flowers, it also had to have one copy of the P allele. So, its genotype must have been Pp.
What is a cross in which an experimenter simultaneously follows the inheritance of two different characters?
Two-factor cross
In two-factor crosses, if link assortment would have been correct, what would be the outcome of a F2 generation? Use the F1 generation of YyRr.
The gametes’ genotypes in the P generation are YR and yr.
If alleles of genes are linked, we cannot have any new combinations meaning that the Punnett square would be:
YR yr for the sperm
YR yr for the eggs
YYRR, YyRr, YyRr, and yyrr is the result of the Punnett square.
Yellow seeds would always be round and wrinkled seeds would always be green with a 3:1 ratio.
Since these alleles of genes are linked, new combinations cannot be created.
This hypothesis was proven incorrect a one point, but there are cases where genes are expressed in pairs therefore creating a link.
What is the hypothesis that proved the linked assortment incorrect?
Independent assortment allows for more combinations of alleles in genes independent of any linkage.
If the two genes are linked, the F1 plants could produce gametes that are only YR or yr. These gametes would combine to produce offspring with the genotypes YYRR (yellow, round), YyRr (yellow, round), and yyrr (green, wrinkled). The ratio of phenotypes would be 3 yellow, round to 1 green, wrinkled. Every F2 plant would be phenotypically like one P-generation plant or the other. None would display a new combination of the parental traits.
However, if the alleles assorted independently, the F2 generation would have a wider range of genotypes and phenotypes.
In this case, the columns and rows will have more combinations since YR and yr are not linked alleles in genes. The example in the book went from YR and yr for both eggs and sperm to TY, Yr, yR, and yr for both eggs and sperm.
In this case, each F1 parent produces four kinds of gametes—YR, Yr, yR, and yr—instead of two, so the square is constructed with four rows on each side and shows 16 possible genotypes. The F2 generation includes plants with yellow, round seeds; yellow, wrinkled seeds; green, round seeds; and green, wrinkled seeds, in a ratio of 9:3:3:1.
What refers to an offspring that is a hybrid with respect to two traits?
Dihybrid
What is the second law of Mendel’s work that states: “The alleles of different genes assort independently of each other during the process that gives rise to gametes.”?
Mendel’s law of independent assostment
[Start 17.2 The Chromosome Theory of Inheritance]
Who addressed that a substance in living cells is responsible for the transmission of hereditary traits?
August Weismann and Karl Nageli in 1883.
This idea challenged other researchers to identify the genetic material. Several scientists, including German biologists Eduard Strasburger and Walther Flemming, observed dividing cells under the microscope and suggested that the chromosomes are the carriers of the genetic material. As we now know, the genetic material is the DNA within chromosomes.
What is an explanation of how the steps of meiosis account for the inheritance patterns observed by Mendel?
What are the fundamental principles? (5)
Chromosome Theory of Inheritance
1. Chromosomes contain DNA, which is the genetic material. Genes are found within the chromosomes. 2. Chromosomes are replicated and passed from parent to offspring. They are also passed from cell to cell during the development of a multicellular organism. 3. The nucleus of a diploid cell contains two sets of chromosomes, which are found in homologous pairs. The maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes. 4. At meiosis, one member of each chromosome pair segregates into one daughter nucleus, and its homolog segregates into the other daughter nucleus. During the formation of haploid Page 356cells, the members of different chromosome pairs segregate independently of each other. 5. Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.
What is the physical location of a gene on a chromosome?
Locus or loci for plural
[Bonus] If two chromosomes are homologous (similar in position, structure, and evolutionary origin but not necessarily in function), can they still be if each chromosome has a different allele, TT and tt, in their loci?
Yes, because they are the same in structure just not in function.
Explain the relationship between the chromosome theory of inheritance and Mendel’s law of segregation.
Two homologous chromosomes prior to DNA replication.
When a cell prepares to divide, the homologs replicate to produce pairs of sister chromatids. Each chromatid carries a copy of the allele found on the original homolog, either T or t.
During meiosis I, the homologs, each consisting of two sister chromatids, pair up and then segregate into two daughter cells. One of these cells has two copies of the T allele, and the other has two copies of the t allele. The sister chromatids separate during meiosis II, which produces four haploid cells. The end result of meiosis is that each haploid cell has a copy of just one of the two original homologs. Two of the cells have a chromosome carrying the T allele, and the other two have a chromosome carrying the t allele at the same locus.
How can we relate the chromosome theory of inheritance to Mendel’s law of independent assortment?
The alleles for seed color (Y or y) and seed shape (R or r) in peas are on different chromosomes (4 individual chromosomes). During metaphase of meiosis I, different arrangements of the two chromosome pairs lead to different combinations of the alleles in the resulting haploid cells.
On the left, the chromosome carrying the recessive y allele has segregated with the chromosome carrying the dominant R allele. On the right, the two chromosomes carrying the dominant alleles (Y and R) have segregated together. Note: For simplicity, this diagram shows only two pairs of homologous chromosomes, though eukaryotic cells typically have several different pairs of homologous chromosomes.
[Extra Information]
When meiosis begins, the DNA in each chromosome has already replicated, producing two sister chromatids. At metaphase I of meiosis, the two pairs of chromosomes randomly align themselves along the metaphase plate. This alignment can occur in two equally probable ways, shown on the two sides of the figure. On the left, the chromosome carrying the y allele is aligned on the same side of the metaphase plate as the chromosome carrying the R allele; Y is aligned with r. On the right, the opposite has occurred: Y is aligned with R, and y is with r. In each case, the chromosomes that aligned on the same side of the metaphase plate segregate into the same daughter cell. In this way, the random alignment of chromosome pairs during meiosis I leads to the independent assortment of alleles found on different chromosomes. For two genes found on different chromosomes, each with two variant alleles, meiosis produces four allele combinations in equal numbers (yR, Yr, YR, and yr), as seen at the bottom of the figure.If a YyRr (dihybrid) plant undergoes self-fertilization, any two gametes can combine randomly during fertilization. Because four kinds of gametes are made, 42, or 16, possible allele combinations are possible in the offspring. These genotypes, in turn, produce four phenotypes in a 9:3:3:1 ratio .This ratio is the expected outcome when a heterozygote for two genes on different chromosomes undergoes self-fertilization.
Let’s suppose that a cell is heterozygous for three different genes (Aa, Bb, and Cc) and that each gene is on a different chromosome. How many different ways can the three pairs of homologous chromosomes align themselves during metaphase I, and how many different types of gametes can be produced?
There are four possible ways that the chromosomes can align, and eight different types of gametes (ABC, abc, ABc, abC, Abc, aBC, AbC, aBc) can be produced.
[Start 17.3 Pedigree Analysis of Human Traits]
Why can geneticists not use the approach of what was done on peas to be used on humans?
For ethical and practical reasons.
What is an examination of human traits over several generations in a family as a way to deduce the pattern of inheritance?
Pedigree analysis
The results of this method may be less definitive than the results of breeding experiments because the small size of human families may lead to large sampling errors. Nevertheless, a pedigree analysis often provides important clues concerning human inheritance.
What two forms do many genes play a role in exist as?
The common allele and a rare allele that has arisen by mutation.
What does pedigree analysis allow us to determine?
Whether the mutant allele is dominant or recessive and to predict the likelihood of an individual being affected.
- What is a genetic disease which involves a mutation in a gene that encodes the cystic fibrosis transmembrane regulator (the CTFR gene)?
- How many Americans of European descent are heterozygous carries of the recessive (disease causing) CFTR allele?
- Who exhibits symptoms of this disease?
- In the human pedigree, how are oldest to newest generations denoted in the modeling?
- What is represented by squares, circles, horizontal and vertical lines?
- Cystic fibrosis (CF)
- 3%
- Individuals who are homozygous for this allele exhibit the disease symptoms, which include abnormalities of the lungs, pancreas, intestine, and sweat glands.
- Roman numerals. I denotes oldest while III would denote newest so on so forth. II would denote middle generation.
- Males are squares, women are circles, horizontal lines directly connecting shapes are mates, vertical lines attached to horizontal lines denote offspring. If shapes are branched off the same horizontal line, they are siblings. Direct connections are only for mates.
Filled shapes are affected, half-filled shapes are presumed heterozygous, and empty shapes are unaffected individuals
Let’s suppose a genetic disease is caused by a mutant allele. If two affected parents produce an unaffected offspring, can the mutant allele be recessive?
No. If two parents are affected with the disease, they would have to be homozygous for the mutant allele if it’s recessive. Two homozygous parents would produce all affected offspring, barring rare mutations. If they produce an unaffected offspring, then the mutant allele is not recessive.
Why does the pedigree indicate a recessive pattern of inheritance for CF?
The answer is that two unaffected individuals can produce an affected offspring. Such individuals are presumed to be heterozygotes (designated by a half-filled symbol).
However, the same unaffected parents can also produce unaffected offspring (depicted by an unfilled symbol), because an individual must inherit two copies of the mutant allele to exhibit the disease.
A recessive mode of inheritance is also indicated by the observation that all of the offspring of two affected individuals are affected themselves. However, for genetic diseases that limit survival or fertility, there are rarely if ever cases where two affected individuals produce offspring.
T/F Most alleles causing human genetic diseases are recessive, but some are known to be dominant.
True.
What is a disease whereas the condition that causes the degeneration of brain cells involved in emotions, intellect, and movement?
Huntington disease.
When does Huntington appear and what specific symptoms are there?
30 to 50 years old, and includes uncontrollable jerking movements of the limbs, trunk, and face; progressive loss of mental abilities; and the development of psychiatric problems.
What observation in a pedigree suggests a dominant pattern of inheritance?
All affected offspring having at least one affected parent suggests a dominant pattern of inheritance.
However, affected parents do not always produce affected offspring. For example, II-6 is a heterozygote that has passed the nondisease-causing allele to his offspring, thereby producing unaffected offspring (III-3 and III-4).
Where are most human genes found?
On paired chromosomes called autosomes, these exclude the sex chromosomes.
Huntington disease is an example of a trait with an autosomal dominant inheritance pattern, whereas cystic fibrosis displays an autosomal recessive pattern. However, some human genes are located on sex chromosomes, which are different in males and females. These genes have their own characteristic inheritance patterns
[17.4 Sex Chromosomes and X-Linked Inheritance Patterns]
What is a distinctive pair of chromosomes that are different in males and females of some species and determine the sex of an individual?
Sex chromosomes
Sex chromosomes are found in many but not all species with two sexes.
If a person is born with only one X chromosome and no Y chromosome, would you expect that person to be a male or a female?
The person is a female. In mammals, the presence of the Y chromosome causes maleness. Therefore, lacking a Y chromosome, a person with a single X chromosome develops into a female.
What gene located on the Y chromosome of mammals plays a key role in the developmental pathway that leads to maleness?
The SRY gene
The X-Y System operates in mammals.
Where does the X-O system operate in and how does it differ from the X-Y system?
It operates in insects.
THe Y chromosome does not determine maleness.
Females in this system have a pair of sex chromosomes and are designated XX. In some insect species that follow the X-O system, the male has only one sex chromosome, the X. In other X-O insect species, such as Drosophila melanogaster, the male has both an X chromosome and a Y chromosome. In all cases, an insect’s sex is determined by the ratio between its X chromosomes and its sets of autosomes.
If a fly has one X chromosome and is diploid for the autosomes (2n), this ratio is 1/2, or 0.5. This fly will become a male whether or not it receives a Y chromosome. On the other hand, if a diploid fly receives two X chromosomes, the ratio is 2/2, or 1.0, and the fly becomes a female.
