1. Extensions on Mendelian Genetics

2. 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?

3. Cystic Fibrosis

4. 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

5. Color Blindness Recessive trait on the X chromosome
Females need two copies to express the trait
Males only need one copy

6. 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.

7. 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?

8. Complex Inheritance Patterns
Sometimes, phenotype results as an interaction between both alleles
Incomplete dominance
Codominance

9. Incomplete Dominance Both traits are mixed in a heterozygous offspring
Example: four o’clock plant
Homozygous red x homozygous white results in pink offspring

10. 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?

11. Codominance Both alleles are expressed completely
Homozygous red crossed with homozygous white would result in red and white spotted offspring
Example: roan cows

12. 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?

13. Polygenic Traits Traits produced by more than one gene
Skin color, eye color, blood type
Blood type is polygenic and codominant!

14. 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.

15. Blood type Practice
What are the expected genotypes and phenotypes of the offspring if the father is type A heterozygous and the mother is type B heterozygous.
What are the expected genotypes and phenotypes of the offspring if the father is type O and the mother is type AB?
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).

16. Pedigrees
Pedigrees help trace phenotypes and genotypes in a family

17. 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

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

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

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

21. Hemophilia in the Royal Family

22. 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

23. Practice

27. 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)