Patterns of Inheritance Flashcards
Heredity
is the transmission of traits from one generation to the next.
Genetics
is the scientific study of heredity.
Gregor Mendel
began the field of genetics in the 1860s,
deduced the principles of genetics by breeding garden peas, and
relied upon a background of mathematics, physics, and chemistry.
In 1866, Mendel
correctly argued that parents pass on to their offspring discrete “heritable factors” and
stressed that the heritable factors (today called genes), retain their individuality generation after generation.
a character
A heritable feature that varies among individuals, such as flower color, is called a character.
trait
Each variant for a character, such as purple or white flowers, is a trait.
True-breeding
varieties result when self-fertilization produces offspring all identical to the parent.
hybrids
The offspring of two different varieties are hybrids.
The cross-fertilization is a hybridization, or genetic cross.
True-breeding Parental plants are the P generation.
monohybrid cross
A cross between two individuals differing in a single character is a monohybrid cross.
Mendel developed 4 hypotheses
- Alleles are alternative versions of genes that account for variations in inherited characters.
- For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.
A homozygous genotype has identical alleles.
A heterozygous genotype has two different alleles. - If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.
- The phenotype is the appearance or expression of a trait.
The genotype is the genetic makeup of a trait.
A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.
A Punnett square
A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.
A locus (plural, loci)
the specific location of a gene along a chromosome.
For a pair of homologous chromosomes, alleles of a gene reside at the same locus.
Homozygous individuals have the same allele on both homologues.
Heterozygous individuals have a different allele on each homologue.
A dihybrid cross
A dihybrid cross is a mating of parental varieties that differ in two characters.
law of independent assortment
Mendel
suggested that the inheritance of one character has no effect on the inheritance of another,
suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and
called this the law of independent assortment.
A testcross
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.
A testcross can show whether the unknown genotype includes a recessive allele.
Mendel used testcrosses to verify that he had true-breeding genotypes.
The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.
Wild-type traits
Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.
The inheritance of human traits follows Mendel’s laws.
A pedigree
shows the inheritance of a trait in a family through multiple generations,
demonstrates dominant or recessive inheritance, and
can also be used to deduce genotypes of family members.
Inherited human disorders show either
recessive inheritance in which
two recessive alleles are needed to show disease,
heterozygous parents are carriers of the disease-allele, and
the probability of inheritance increases with inbreeding, mating between close relatives.
dominant inheritance in which
one dominant allele is needed to show disease and
dominant lethal alleles are usually eliminated from the population.
Dominant human disorders include
achondroplasia, resulting in dwarfism, and
Huntington’s disease, a degenerative disorder of the nervous system.
The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is
recessive and
carried by about 1 in 31 Americans
complete dominance
Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.
incomplete dominance
For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which
neither allele is dominant over the other and
expression of both alleles occurs.
Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.
One example of incomplete dominance in humans is hypercholesterolemia, in which
dangerously high levels of cholesterol occur in the blood and
heterozygotes have intermediately high cholesterol levels.
codominance
The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.
In codominance,
neither allele is dominant over the other and
expression of both alleles is observed as a distinct phenotype in the heterozygous individual.
AB blood type is an example of codominance.
Pleiotropy
occurs when one gene influences many characteristics.
Sickle-cell disease is a human example of pleiotropy. This disease
affects the type of hemoglobin produced and the shape of red blood cells and
causes anemia and organ damage.
Sickle-cell and nonsickle alleles are codominant.
Carriers of sickle-cell disease are resistant to malaria.
polygenic inheritance
Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
Human skin color is an example of polygenic inheritance.
Many characters result from a combination of heredity and the environment. For example……..
skin color is affected by exposure to sunlight,
susceptibility to diseases, such as cancer, has hereditary and environmental components, and
identical twins show some differences.
Only genetic influences are inherited.
The chromosome theory of inheritance states that
genes occupy specific loci (positions) on chromosomes and
chromosomes undergo segregation and independent assortment during meiosis.
Mendel’s laws correlate with chromosome separation in meiosis.
The law of segregation depends on separation of homologous chromosomes in anaphase I.
The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.
linked genes
ateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which
are located close together on the same chromosome and
tend to be inherited together.
recombination frequency
Crossing over between homologous chromosomes produces new combinations of alleles in gametes.
Linked alleles can be separated by crossing over, forming recombinant gametes.
The percentage of recombinants is the recombination frequency
Many animals have a pair of sex chromosomes
designated X and Y,
that determine an individual’s sex.
In mammals,
males have XY sex chromosomes,
females have XX sex chromosomes,
the Y chromosome has genes for the development of testes, and
an absence of the Y allows ovaries to develop.
In some animals, environmental temperature determines the sex.
For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.
Global climate change may therefore impact the sex ratio of such species.
Sex-linked genes
are located on either of the sex chromosomes.
The X chromosome carries many genes unrelated to sex.
The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.
Most sex-linked human disorders are
due to recessive alleles and
seen mostly in males.
A male receiving a single X-linked recessive allele from his mother will have the disorder.
A female must receive the allele from both parents to be affected.
Recessive and sex-linked human disorders include
hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,
red-green color blindness, a malfunction of light-sensitive cells in the eyes, and
Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.
