1. Exploring Unlinked Genes to Sex-Linked Genes
Simple Genetics
Exploring Unlinked Genes to Sex-Linked Genes

2. Overview Autosomal Genes Dihybrid Crosses Sex-Linked Genes
Punnett squares
Dihybrid Crosses
Bigger Punnett squares...
Sex-Linked Genes
Thank your mom

3. The Allele
Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.
Phenotype: The observable physical and physiological traits of an organism, which are determined by it’s genetic makeup.
Genotype: The genetic makeup, or set of alleles, of an organism.
Alleles are found on chromosomes
One per chromosome
Separated in meiosis
Maternal vs. Paternal chromosomes
Dominant vs. Recessive
Codominance vs. incomplete dominance

4. H-words Homozygous: Having two identical alleles for a given trait
Dominant vs. Recessive
Heterozygous: Having two different alleles for a given trait

5. Mendell (He’s the pea dude)
Independent Assortment: Each pair of alleles assort independently of each other during gamete formation
Law of Segregation: Two allele pairs segregate into different gametes

6. Biology’s favorite shape
The Punnett Square: A diagram used in the study of inheritance to show the predicted genotypic results of random fertilization in genetic crosses between individuals of a known genotype
Shows genotypes of possible offspring of two parents

7. Monohybrid Crosses A cross where only one trait is being explored
Based on when a single set of chromosomes is separated
Diagramed by a 2 x 2 Punnett Square
Define alleles
Set up square
Cross
Sample: A heterozygous green pea plant is crossed
with a yellow pea plant. Show the cross.
A a
A = green
a = yellow a
Aa aa

8. Sample Problems
Two pea plants heterozygous for pod color are crossed. Define allele notation and diagram a Punnett Square for this cross.
A homozygous dominant red flower is crossed with a homozygous recessive white flower. They produce all pink flowers, what is the reason for this? What would be produced if a pink offspring was crossed with another pink offspring? Diagram a Punnett Square.
A blue eyed man and brown eyed women have four children, none of which have blue eyes. Does blue eyes appear to be dominant or recessive?

9. Dihybrid Crosses A cross where two genes are explored simultaneously
Physically based on alleles located on two sets of chromosomes, which then segregate into gametes.
Genes must be unlinked (not on the same chromosome)
9:3:3:1
Requires a 4 x 4 Punnett Square
Determine possible gamete formations
Set up square
Cross!
Remember this? It’s a dihybrid cross!

10. 9:3:3:1, the magic number
Refers to the phenotypic ratio expected from a normal dihybrid cross of two heterozygous parents. This was devised by Mendell.
9: the expected number of offspring with all dominant traits
3: the expected number of offspring with one dominant and one recessive trait
3: the reciprocal of the first ‘3’
1: expected number of offspring with all recessive traits

11. Sample Problem Cross: AaBb x AaBb
Possible gametes (Parent 1): AB, Ab, aB, ab
Possible gametes (Parent 2): AB, Ab, aB, ab
AB Ab aB ab AB Ab aB ab

12. Practice Problems
Two dogs heterozygous for brown fur and blue eyes are crossed. Diagram this cross. What is the probability of the dogs having a white fur-green eyed dog?
A dog true-breeding for brown fur and blue eyes is crossed with a dog true-breeding for white fur and green eyes. Diagram the cross. What is the probability that these parents will have a white fur-green eyed offspring?

13. Sex-Linkage The miracle of the Y chromosome
Does not include alleles found on it’s homologous X chromosome
Males have increased susceptibility to these types of alleles
Two X chromosomes makes a female
one X and one Y makes a male!

14. Crosses for traits that are sex-linked
Also known as x-linked traits
Notation: XA Xa and Y for males
X and Y represent the sex chromosome
When crossing, X X will always represent female and the X Y will always represent male.
Carrier: An individual who is heterozygous for a specific trait, thusly it affects their offspring, but not their phenotype.

15. 1/2 Chance of hemophilic daughter D:
Sample Problem!
Hemophilia, a sex linked disorder, causes difficulty in blood clotting, it is a recessive disorder. A hemophiliac man marries a woman whose father was also a hemophiliac, but she is healthy. What is the probability that this couple will have a hemophiliac daughter?
Determine genotypes of parents
Remember, this is a monohybrid cross, set up a square
Preform the cross (do not forget correct notation)
XH Xh
Father: XhY
Mother: XHXh
1/2 Chance of hemophilic daughter D: Xh Y
XHXh XhXh
XHY XhY
XH= healthy
Xh= hemophiliac

16. A Last Problem...
A healthy man and a healthy women have a hemophilic child. The man believes this is evidence that his wife has had an affair. What is the genotype of the man and his wife? Is this evidence of an affair?