1. If you do, your chromosomes could look like this…
Do you have dimples?
Dimples are caused when a person inherits at least 1 dominant allele from the parents.
If you do, your chromosomes could look like this…
Dimples
No dimples OR

2. Mendelian Genetics

3. Key Terms in Mendelian Genetics
Phenotype: observable traits
Brown eyes, yellow flower, dimples
Genotype: the actual combination of alleles inherited
Examples: BB, dd, Ff
Phenotype: dimples
This is the trait that is expressed.
What could be this girl’s genotype?
She could be DD or Dd.

4. A person’s genotype could be…
Homozygous: having 2 identical alleles for a trait
TT or tt
“homo” means “same”
Heterozygous: having 2 different alleles for a trait Tt
“hetero” means “different”
This person has a widow’s peak.
Genotype: could be Aa or AA
This person could be homozygous dominant (AA) or heterozygous (Aa) for the widow’s peak trait.

5. Let’s say Kourtney & Scott decide to have a baby
Let’s say Kourtney & Scott decide to have a baby. Will the baby have a widow’s peak?
We can use our knowledge of genetics & meiosis to make a prediction!

6. Meiosis & Sexual Reproduction
During meiosis, our bodies make egg & sperm cells.
Egg & sperm cells contain ½ the amount of DNA as other cells.
WHY?

7. Meiosis & Sexual Reproduction
Each parent donates 23 chromosomes to a baby, for a total of 46 chromosomes (23 pairs).
Let’s follow 1 gene – the gene for widow’s peak.
A = widow’s peak, a = no widow’s peak A a

8. Kourtney & Scott’s Genotypes
Kourtney’s Phenotype: widow’s peak What would be her genotype? Could be Aa or AA. Let’s assume she is heterozygous, Aa.
Scott’s Phenotype: no widow’s peak
What would be his genotype?
MUST be homozygous recessive, aa.

9. During meiosis, these 2 alleles separate into different egg cells.
Kourtney is heterozygous, meaning she carries a dominant & a recessive allele (Aa).
During meiosis, these 2 alleles separate into different egg cells.
SO, there is a 50% chance that she will pass on the widow’s peak allele (A) to her baby.
There’s also a 50% chance she will pass on the normal allele (a) to her baby. OR

10. Scott is homozygous recessive, meaning he carries 2 copies of the recessive allele (aa).
The ONLY allele that Scott can pass down to his baby is the recessive allele (no widow’s peak), because that’s all he has!

11. a a A Aa Aa a aa aa Punnett Squares
Diagrams we can draw to help us predict the genotypes & phenotypes of offspring. a a A Aa Aa a aa aa

12. Start with a blank box with 4 empty squares.
Add the genotype of each parent to the top & left side of the box (outside). Father is on top & mother is on the left side. a a A a

13. a a A Aa Aa a aa aa Fill in the squares.
Remember that each parent only passes on 1 allele to the baby. a a A Aa Aa a aa aa

14. Remember, alleles are a version of a gene.

16. What do the results of the Punnett square mean?
Kourtney & Scott will have 4 children
Two children will look like Kourtney, & two will look like Scott.
There is chance that a baby will have a widow’s peak, but there is also a chance that the baby won’t have a widow’s peak.

17. A = widow’s peak a = no widow’s peak
What phenotype will the baby have if it inherits these alleles?

18. 50% What are the chances that the baby will have a widow’s peak? 25%
75%
50%
No chance
50%

19. In guinea pigs, long hair (H), is dominant to short hair (h).
A heterozygous long haired pig mates with a short haired pig. Complete a punnett square to determine what the genotype and phenotype ratios would be for their offspring.

20. In fruit flies, long wings (L) are dominant to short wings (l).
A true-breeding long wing female mates with a true-breeding short wing male. Complete a punnett square to determine what the genotype and phenotype ratios would be for their offspring.