Heredity Flashcards
Heredity
- is the passing of traits from parents to offspring. These traits can be passed down
sexually (mating in animals) or asexually (binary
fission in bacteria).
Genome
- all the DNA within a cell.
Gene
- sequence of DNA that codes for a trait.
Locus
- location of a gene on a chromosome.
Plural is gene loci.
Allele
- one variation of a gene. Alleles are
found at the same loci on both chromosomes
in a homologous pair.
Wild-type allele
- normal allele that is most
common in nature. Can turn into a mutant
allele.
Mutation
- heritable change in DNA.
Genotype
- genetic composition of an
organism.
Phenotype
- observable traits that result from a genotype.
Dominant alleles
- mask the expression of
recessive alleles. Typically represented by
uppercase letters (“A”).
Recessive alleles
- only show up in a
phenotype if dominant alleles are not present.
Typically represented by lowercase letters (“a”).
Homologous pairs
- two different copies of
the same chromosome in a diploid organism.
One from each parent. Each copy is very
similar, except for minor nucleotide variations
that generate unique alleles.
Heterozygous
- one dominant allele and one
recessive allele in its homologous pair.
Homozygous
- same allele in both homologs.
Can be homozygous dominant or
homozygous recessive.
Hemizygous
- only one allele is present. For example, men only have one X and one Y chromosome (not homologous), which contain
hemizygous genes.
Penetrance
- proportion of individuals who have the phenotype associated with a specific
allele. Can be complete penetrance or incomplete penetrance. As shown below, Bb individuals all have brown eyes only when
there is complete penetrance.
Expressivity
- describes the degree of a certain phenotype for a given genotype. All of the
children of this couple have genotype Hh for
medium thick hair, but because of expressivity,
just how medium thick (or medium thin) the
hair is varies.
Incomplete dominance
- is when one allele is not
completely expressed over its paired allele. The
heterozygous will have an intermediate state. (Ex.
red x white = pink).
Codominance
- is when the heterozygous genotype expresses both alleles. (Ex. red x white = red + white spots).
Multiple alleles
- describe when there are more allele options than just two. (Ex. ABO blood typing
- A, B, O alleles).
Epistasis
- is when one gene affects the expression
of a different gene. (Ex. baldness gene covers up
the genes for hair color).
Pleiotropy
- describes when one gene is
responsible for many traits. (Ex. cystic fibrosis is a
disease with many symptoms caused by a single
gene).
Polygenic inheritance
- is when many genes are
responsible for one trait. This gives the trait
continuous variation. (Ex. height, a single trait
affected by many genes).
Haploinsufficiency
- occurs when one copy of the gene is lost or nonfunctional and the expression of
the remaining copy is not sufficient enough to
result in a normal phenotype. It can result in an
intermediate phenotype.
Haplosufficiency
- describes when the remaining copy of the gene is sufficient enough to result in a normal phenotype.
Proto-oncogenes
- are genes that can become
oncogenes (cancer-causing genes) due to gain-of-function mutations.
-Gain-of-function
mutations can cause too much protein to be
made or production of an over-active protein;
Cancerous growth occurs as a result. Proto-oncogenes are normally involved in cell cycle
control.
one hit hypothesis
- proto-oncogenes follows this, which states that a gain-of-function mutation in
one copy of the gene turns it into an oncogene.
Tumor-suppressor genes
- are genes that become
cancerous as a result of loss-of-function mutations, because they are normally needed to suppress cancerous growth. - follows 2 hit hypothesis
Two hit hypothesis
- which states that a loss-of-function mutation in both copies of the gene are needed to make it cause cancer. Thus, tumor-suppressor genes are haplosufficient.
Null alleles
- come from mutations that cause the alleles to lack normal function. Tumor-suppressor genes have null alleles when they become
cancer-causing.
p53
- is an important tumor-suppressor gene
that is known as the guardian of the cell. It is
upregulated to prevent cells from becoming
cancerous.
p21
- is another tumor-suppressor gene that inhibits phosphorylation activity in order to decrease rampant cell division.
Retinoblastoma gene (RB)
- is a tumor-suppressor gene that codes for a retinoblastoma protein, which prevents excessive cell growth during interphase.
______ studied genetics and proposed three laws:
1) Gregor Mendel
2) 3 laws: law of dominance, law of segregation, law of independent assortment
Law of dominance
- dominant alleles mask
the expression of recessive alleles. Mendel studied plant height to come to this conclusion.
Law of segregation
- homologous gene copies
separate during meiosis (specifically anaphase
I). Thus, Aa individuals will produce gametes
with “A” or “a” alleles.
Law of independent assortment
- homologous chromosomes line up independently during metaphase I of meiosis
so that alleles separate randomly (this increases genetic variability). - Metaphase II is
different, during which sister chromatids are pulled apart instead. The law of independent assortment can produce 2^23 (23
homologous chromosome pairs split).
Under the law of independent assortment, if we consider a _________
(haploid number is 3), the 6 chromosomes could
assort with:
1) 6 chromosome diploid organism
Nondisjunction
- is the improper segregation of chromosome pairs during anaphase; it produces
daughter cells with an incorrect number of
chromosomes.
1) single nondisjunction during Meiosis I
2) single nondisjunction during Meiosis II
3) Single nondisjunction of sister chromatids
during mitosis
single nondisjunction during Meiosis I
46 chromosomes in diploid parent cell → 24, 24, 22, 22 chromosomes in haploid daughter cells
Single nondisjunction of sister chromatids
during meiosis II
46 chromosomes in diploid parent cell → 24, 22, 23, 23 chromosomes in haploid daughter cells
Single nondisjunction of sister chromatids
during mitosis
- 46 chromosomes in diploid parent cell → 47, 45 chromosomes in diploid daughter cells
Aneuploidy
- refers to an abnormal number of chromosomes in the daughter cells. After
fertilization, trisomy (3 chromosome copies) or
monosomy (1 chromosome copies) can occur. - Disomy refers to a normal diploid cell.
Down syndrome
- is a trisomy of chromosome #21 (each diploid cell has 47 chromosomes total).
Turner syndrome
- is a monosomy of the X
chromosome in females (each diploid cell has 45
chromosomes total). Affected individuals have
physical abnormalities and sterility.
Klinefelter’s syndrome
- is a trisomy of the sex
chromosomes in males, giving them XXY (each
diploid cell has 47 chromosomes total). Individuals usually have disorders in intellectual, physical, and reproductive development.
Trisomy X (triple X syndrome)
- is a trisomy of the
sex chromosomes in females, giving them XXX (each diploid cell has 47 chromosomes total).
Learning disabilities (learning, attention, etc.) are
often observed in these individuals.
Cross
- refers to when two organisms are mated
to produce offspring.
Test-cross
- an individual of unknown
genotype with one that is homozygous recessive. By
looking at the offspring from a test-cross, we can
determine the unknown genotype.
True-breeding organisms
- are homozygous for all
the traits of interest.
F1 generation (aka filial 1 hybrid)
- the first generation cross between true-breeding parents with different alleles. The offspring are all
heterozygous.
F2 generation (aka filial 2 hybrid)
- is the second generation cross between the
heterozygous offspring from the F1 generation.
This is where Mendel’s three laws can be studied.
If these two generations are studied under ______, then only a single gene is examined. In the F2 generation, the genotype ratio (AA:Aa:aa) should be (1:2:1) and the phenotype
ratio (dominant:recessive) should be (3:1).
1) monohybrid crosses
On the other hand, a ______ examines the
inheritance of two genes on separate chromosomes. Although the genotype ratio is complex in the F2 generation, just remember that the phenotype ratio (both dominant:one
dominant and one recessive:one dominant and
one recessive:both recessive) should be (9:3:3:1).
1) dihybrid cross
Punnett squares
- are used to visualize these
crosses but are too complex for dihybrid crosses.
Thus, one-gene cross ratios can be used to solve
these questions faster.
Homozygous x homozygous
= 1/1 AA or 1/1 Aa or 1/1 aa
Homozygous x heterozygous
= = 1⁄2 AA (or aa) and 1⁄2 Aa
Heterozygous x heterozygous
= = 1⁄4 AA, 1⁄2 Aa, 1⁄4 aa
Multiple-locus
- crosses can then be solved using these single allele crosses. As shown below, RrYy individuals cross with each other. The Rr single
cross probabilities can be multiplied with the Yy
single cross probabilities to get the dihybrid offspring probabilities shown on the right.
Pedigree charts
- are used to track inherited traits over many generations to see inheritance patterns. Females are represented by circles, and males are represented by squares. Individuals affected by the trait in question are shaded;
unaffected individuals are not shaded.
The logic goes like this; since the male is affected,
we know that he can be ______ or
_______ . However, since his father is unaffected, the male could not have received an “affected” allele from his father. Thus, this
individual must be heterozygous. The single
dominant allele came from his mother.
1) heterozygous
2) homozygous dominant
Crossing over
- also creates genetic diversity and
occurs during prophase I of meiosis. Homologous
chromosomes join together to form tetrads (aka
bivalents) and exchange genetic material at points
referred to as chiasmas. Afterwards, genetically
unique chromatids are produced as a result of
crossing over.
Recombinant gametes
- describe the gametes that
receive the genetically unique chromatids (new combination of alleles), while non-recombinant gametes refer to the gametes that receive
parental chromatids (alleles match parent’s
alleles).
Linked genes
- are found close together on the same chromosome. By looking at recombination
frequencies, we can deduce the relative distance
between these genes.
One ______ is defined as the chromosomal
distance that would allow 0.01 crossover events
per generation. ______ would mean 0.2
crossover events occur between the two genes per
generation, or that there is a 20% chance of
recombination.
1) map unit
2) 20 Map unit
Recombination frequencies of _________ mean that the two genes are linked. A random assortment of ______ have 50%
recombinant progeny.
1) less than 50%
2) unlinked genes
Linkage maps
- are tables that are used to determine the probability of inheritance. Linkage
maps use map units to infer the distance between
genes on a chromosome.
A haplotype
- is a group of genes that are usually inherited together because they are located in
close proximity to each other.
Sex-linked traits
- come from genes located on the sex chromosomes. Most sex-linked disorders have
X-chromosome linkage.
3 types of sex-linked traits
1) X-linked dominant
2) X-linked recessive
3) Y-linked
X-linked dominant
- dominant inheritance on
the X chromosome. Any offspring (male or
female) that receive the affected allele will end
up with the disorder.
X-linked recessive
- recessive inheritance on
the X chromosome. For males, only one affected allele is needed to cause the disorder. For females, two affected alleles are needed to
cause the disorder because females have two X chromosomes. Hemophilia and color-blindness are examples of X-linked
recessive conditions.
Y-linked
- inheritance on the Y chromosome.
Can only be passed from father to son. Will
always be expressed whether it is dominant or
recessive because males only have one Y
chromosome.
Genomic imprinting
- refers to genes that are
expressed depending on parental origin and are
influenced by epigenetic factors. These genes are
different from sex-linked traits because they can
come from autosomal chromosomes (non-sex
chromosomes) as well.
X-inactivation
- is the process by which one of a female’s X chromosomes is inactivated, forming a
Barr body and preventing excess transcription.
However, a female carrier may become an
affected individual for a disease if her unaffected
X chromosome with a normal wild-type allele is
inactivated, leaving behind a recessive allele that is
not covered up.
Epigenetics
-Epigenetics isthe study of how your behaviors and environment can cause changes that affect the way your genes work. Unlike genetic changes, epigenetic changes are reversible and do not change your DNA sequence, but they can change how your body reads a DNA sequence. Does not involve modifying the genetic code, but instead the regulation of when
genes are expressed. By modifying the DNA with
methylation or acetylation, the same genetic code
can be expressed differently.
Examples of epigenetic changes
1) DNA methylation
2) Histone Acetylation
3) Histone De-acetylation
4) Histone methylation
Epigenetic changes can cause monozygotic twins to have different susceptibilities to the same disease.
DNA methylation
- Decreases gene
expression by the suppression of genes. This
is done through the addition of methyl groups,
recruiting methyl-binding proteins (MBDs) and
preventing transcription factors from binding.
Histone acetylation
- causes gene activation
and formation of euchromatin (easily accessible DNA).
Histone de-acetylation
- causes gene
suppression (think ‘de-activation’) and
formation of heterochromatin (hard to access
DNA).
Histone methylation
- can upregulate or
downregulate gene expression depending on
methyl group location and number.
