1. Biology MCAS Review: Mendelian Genetics

2. Inheritance Vocabulary
Each individual (sexually reproducing) receive one copy of a gene from each parent
Alleles
Dominant
Recessive
Genotype
Homozygous
Heterozygous
Phenotype

3. Punnett Square
The gene combinations that might result from a genetic cross can be determined by drawing a Punnett square
Used to make predictions about what alleles an individual would inherit based on the parents’ genes
Expected vs. Observed results
Possible alleles in male gametes
Possible allele combinations of the offspring
Possible alleles in female gametes

4. Mendel’s Laws of Inheritance
Law of Dominance
--Some alleles are dominant
Always overpowers the recessive allele when together
Represented with a capital letter
EXAMPLE: The purple allele is dominant so Purple flowers = P
--Some alleles are recessive
Always overpowered by the dominant allele when present
Recessive traits are only expressed when there are two recessive alleles present
Represented with a lowercase letter (letter is the same as the dominant letter)
EXAMPLE: The white allele is recessive so White flowers = p

5. Mendel’s Laws of Inheritance
Law of Segregation
When gametes are formed, the alleles for each gene in an individual separate from one another during meiosis.
As a result, gametes only carry only allele for each trait.

6. Mendel’s Laws of Inheritance
Law of Independent Assortment
As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another independently of genes pairs on other chromosomes
So…..genes that segregate independently of one another do NOT affect the inheritance of each other during gamete formation….

7. Possible Chromosome Combinations1 2 3
Possible Chromosome Combinations or
Maternal and paternal chromosomes are randomly aligned or or

8. Inheritance Patterns 1. Complete Dominance
Dominant allele completely overpowers recessive alleles when present together

9. Complete Dominance Monohybrid Cross Example
If you cross a heterozygous plant with a homozygous recessive plant, how many of the offspring will show the recessive phenotype?

10. Complete Dominance Dihybrid Cross Example
One organism heterozygous for both traits is crossed with another individual homozygous recessive for both traits. Phenotypic ratio of offspring?

11. 2. Incomplete Dominance
One allele is not completely dominant over the other…
Heterozygote shows intermediate phenotype

12. 3. Codominance Two alleles are dominant at the same time
Both phenotypes are expressed simultaneously if both present at the same time

13. 4. Multiple Alleles More than 2 alleles for one particular gene
Multiple phenotypes
can result

14. Polygenic Traits
More than one gene (and many alleles!) interact to produce one phenotype
Continuous variation
A continuous range of small differences in a given trait among individuals
The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in that trait

15. 6. Sex-linked (X-linked) traits
Section 14-2
Inheritance of genes found on sex chromosomes
XX in females, XY in males
Traits are more common in males than in females…why?
Go to Section:

16. Figure 14-13 Colorblindness
Section 14-2
Father
(normal vision)
Normal vision
Colorblind
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Mother (carrier)
Daughter
(carrier)
Son
(colorblind)
Go to Section:

17. Pedigree: Tool used to track the inheritance of a trait through generations
A square represents a male.
A circle represents a female.
A vertical line and a bracket connect the parents to their children.
A horizontal line connecting a male and female represents a marriage.
A half-shaded circle or square indicates that a person is a carrier of the trait.
A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait.
A completely shaded circle or square indicates that a person expresses the trait.
Go to Section:

18. This pedigree shows the inheritance of attached ear lobes
This pedigree shows the inheritance of attached ear lobes. Which parent has attached ear lobes?
2. How many children do the parents have? Which child has attached ear lobes?
Which child is married? Does this child’s spouse have attached ear lobes? Do any of this child’s children have attached ear lobes?
Free earlobes are dominant over attached earlobes. Determine the genotypes of the individuals below (to the best of your ability based on the information provided to you in the pedigree).