Chapter 7 Review Carrier Sex-linked genes X chromosome inactivation Incomplete dominance Codominance Polygenic traits Epistasis Pedigrees

What is a carrier? Some human genetic disorders are caused by recessive alleles on autosomes. Both recessive alleles must be present for a person to have the disorder. Parents who are heterozygous – they have a dominant AND recessive allele – will NOT have the disorder, but will carry the recessive allele, and can pass it on to their offspring.

What are sex-linked genes? Genes that are located on the sex chromosomes are called sex-linked genes. The X chromosome has many genes that affect many traits. Because males have only one copy of each type of sex chromosome, they express ALL of the alleles on both chromosomes.

What is X chromosome inactivation? In each cell of female mammals, one of the X chromosomes is randomly turned off by a process called X chromosome inactivation. Female calico cats have white fur, as well as alleles for black or orange fur on their X chromosomes. Those alleles are expressed randomly in cells across the cat’s body. As a result, its coat is a mixture of color patches. Because male cats only have one X chromosome, they have white fur and one sex-linked gene for either orange or black fur.

What is X chromosome inactivation? Female calico cats have white fur, as well as alleles for black or orange fur on their X chromosomes. Those alleles are expressed randomly in cells across the cat’s body. As a result, its coat is a mixture of color patches. Because male cats only have one X chromosome, they have white fur and one sex-linked gene for either orange or black fur.

What is incomplete dominance? The pea flowers that Gregor Mendel observed were either purple or white. One allele was dominant. That means the dominant allele codes for a certain protein and the recessive allele codes for a variation of the protein that has little or no effect. But in many cases, a phenotype comes from more than just one gene, and many genes have more than just two alleles.

What is incomplete dominance? Sometimes, alleles show incomplete dominance, in which a heterozygous phenotype (Aa) is somewhere between the homozygous dominant phenotype and the homozygous recessive phenotype. Neither allele is completely dominant nor completely recessive.

What is codominance? Sometimes, both alleles of a gene are expressed completely. Codominance is when both traits are fully and separately expressed.

What is codominance? Example: Blood Three alleles: A B O A Blood Type Genotypes A IA IA or IA i B IB IB or IB i AB IA IB O i i

Incomplete Dominance vs Codominance

What are polygenic traits? Traits that are produced by two or more genes are called polygenic traits. Skin color: 4 genes Eye color: 3 genes

What is epistasis? Sometimes a gene can interfere with the expression of other genes. This is called epistasis. In albinism, a single epistatic gene interferes with the expression of other genes. Albinism is a recessive genetic disorder in which the body is unable to produce an enzyme (tyrosinase) necessary for the production of melanin (dark color to hair, skin, scales, eyes, and feathers).

The environment interacts with genotype Phenotype is usually a mixture of genes and environment. For example, one twin might get more nutrients than the other because of its position in the mother’s uterus. This difference can result in height and size differences. Also, twins raised in environments with different nutrition and health care often differ in height and other physical traits.

What is a pedigree? A pedigree is a diagram showing genetic relationships between members of a family. It is used to analyze patterns of inheritance for specific genetic traits.

What is a karyotype? A karyotype is a picture of all of the chromosomes in a cell. Karyotypes can show chromosomal disorders, such as the deletion of a chromosome or an extra chromosome.

How to interpret a pedigree chart Determine if the pedigree chart shows an autosomal or X- linked disease. If most of the males in the pedigree are affected, then the disorder is X-linked If it is a 50/50 ratio between men and women the disorder is autosomal.

How to interpret a pedigree chart Determine whether the disorder is dominant or recessive. If the disorder is dominant, one of the parents must have the disorder. If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.

How to interpret a pedigree chart Rules of Inheritance Autosomal Recessive Appears in both sexes with equal frequency Trait tend to skip generations Affected offspring are usually born to unaffected parents

How to interpret a pedigree chart Rules of Inheritance Autosomal Dominant Appears in both sexes with equal frequency Both sexes transmit the trait to their offspring Does not skip generations Affected offspring must have an affected parent unless they posses a new mutation

How to interpret a pedigree chart Rules of Inheritance X-Linked Dominant Both males and females are affected; often more females than males are affected Does not skip generations. Affected sons must have an affected mother Affected daughters must have either an affected mother or an affected father Affected fathers will pass the trait on to all their daughters

How to interpret a pedigree chart Rules of Inheritance X-Linked Recessive More males than females are affected Affected sons are usually born to unaffected mothers, thus the trait skips generations It is never passed from father to son All daughters of affected fathers are carriers

If individuals can roll their tongues, then they have at least one Dominant allele. If individuals cannot roll their tongue, then the are homozygous recessive. Label all unaffected individuals as homozygous recessive. ALLELES T = Tongue roller t = Non tongue roller t t t t t t t t t t t t t t t t

T t t t t t T t t t t t T t T t T t t t t t t t t t T t or T T T t or ALLELES T = Tongue roller t = Non tongue roller If individuals cannot roll their tongue, then the are homozygous recessive. Label all unaffected individuals as homozygous recessive. T t t t t t T t t t t t T t T t T t t t t t t t t t T t or T T T t or T T T t T t