Extensions on Mendelian Genetics

Genotype vs. Phenotype

• Since an individual has two copies of each gene, a heterozygous individual carries the recessive gene without showing it.

• This is referred to as a carrier.• Some human disorders are caused by recessive

alleles (ex: cystic fibrosis).• Other disorders, such as Huntington’s disease,

are caused by dominant alleles.• Which one do you think is more common?

Cystic Fibrosis

Sex-Linked Genes

• Genes located on sex chromosomes (X or Y) are called sex-linked genes.

• Y chromosome is smaller and has fewer genes– Mostly just for sex determination

• Females are XX; males are XY• Males express all genes from both

chromosomes because there is not another copy to “mask” recessive traits

Color Blindness

• Recessive trait on the X chromosome• Females need two copies to express the trait• Males only need one copy

Practice

• The gene for colorblindness is carried on the X chromosome and is recessive. A colorblind man marries a woman of normal vision. They have a daughter who is colorblind.– What are the genotypes of each parent?– What is the daughter’s genotype?– Show a Punnett square predicting the possible

genotypes and phenotypes that could be produced if the couple has another child.

Practice

• Hemophilia is carried on the X chromosome and is recessive to normal blood type. Cross a carrier female to an affected man.– What percentage of the offspring will be carrier

females?– What percentage will be normal males?– What percentage will be hemophiliac females?– How many genotypes are possible among the

offspring?

Complex Inheritance Patterns

• Sometimes, phenotype results as an interaction between both alleles– Incomplete dominance– Codominance

Incomplete Dominance

• Both traits are mixed in a heterozygous offspring

• Example: four o’clock plant– Homozygous red x homozygous white results in

pink offspring

Incomplete Dominance Practice

• In radishes, the gene that controls color exhibits incomplete dominance. Pure-breeding red radishes (RR) crossed with pure-breeding white (WW) radishes make purple radishes (RW). What are the genotypic and phenotypic ratios when you cross a purple radish with a white radish?

Codominance

• Both alleles are expressed completely– Homozygous red crossed with homozygous white

would result in red and white spotted offspring– Example: roan cows

Codominance Practice

In shorthorn cattle hair color is decided by a pair of codominant traits. R is the allele for red hair color and W is the allele for white hair color. A cow with the heterozygous genotype is roan in color, meaning its coat contains both white and red hairs.What would be the expected phenotypes of the offspring if a white cow is bred with a roan cow?

Polygenic Traits

• Traits produced by more than one gene– Skin color, eye color, blood type– Blood type is polygenic and codominant!

Blood Type

• A man with type AB blood marries a woman with type B blood. Her mother has type O blood. List the expected phenotype & genotype frequencies of their children.

Blood type Practice1. What are the expected genotypes and phenotypes of

the offspring if the father is type A heterozygous and the mother is type B heterozygous.

2. What are the expected genotypes and phenotypes of the offspring if the father is type O and the mother is type AB?

3. Ralph has type B blood and his wife Rachel has type A blood. They are very shocked to hear that their baby has type O blood, and think that a switch might have been made at the hospital. Can this baby be theirs? Explain why or why not (use a Punnett square to help).

Pedigrees

• Pedigrees help trace phenotypes and genotypes in a family

Pedigrees

• Pedigrees can be analyzed to help you answer several different questions:–Is this trait dominant or recessive?• Recessive traits will be less common

–Is this trait autosomal or sex linked?• Autosomal traits should affect equal

numbers of males and females• Sex linked traits will affect more males

Is this trait dominant or recessive?Sex-linked or autosomal?

Is this trait dominant or recessive?Sex-linked or autosomal?

Is this trait dominant or recessive?Sex-linked or autosomal?

Hemophilia in the Royal Family

Practice

• What are the genotypes of individuals 1 and 2?

• How many of their offspring are colorblind?

• Are any carriers?• What is the genotype of

individual 3? How do you know?

1 2

3

Practice

Karyotyping

• A karyotype is a picture of all the chromosomes in a cell

• Chromosome mapping can help diagnose genetic disorders such as Down syndrome (having an extra copy of the 21st chromosome) or Klinefelter’s (XXY)