Understanding Genetics: Genes, Alleles & Inheritance

1. Genetics and Heredity

2. Genes and Alleles • Chromosomes contain DNA • Sections of DNA are called genes • You have a gene that determines every trait (characteristic) you have • Each gene has a pair of alleles (two) • The alleles control what trait that gene expresses

3. Dominant and Recessive Alleles • Basic traits have two alleles, one dominant allele and one recessive allele. • Ex) eye color in a population of bunnies • Brown eyes are dominant • Blue eyes are recessive • Alleles are represented by letters • In most cases, the dominant allele is a CAPITAL letter and the recessive allele is a lowercase letter. • Ex) Brown = B, Blue = b

4. Dominant and Recessive Alleles • A dominant allele will be expressed (shown) over a recessive allele. • Ex) Possible combinations of alleles for bunny eye color: • BB  brown eyes – both alleles code for brown eyes • Bb  brown eyes – the brown allele dominates the blue allele and the bunny has brown eyes • bb  blue eyes – both alleles code for blue eyes

5. Homozygous and Heterozygous • Combinations of the same allele are called “homozygous”. • Combinations of different alleles are called “heterozygous” • Ex) Possible combinations of alleles for bunny eye color: • BB  homozygous dominant • Bb  heterozygous • bb  homozygous recessive

6. Genotype and Phenotype • The specific combination of alleles is called the genotype. Genotype refers to all hereditary information whether it is expressed or not. • The outward appearance of a trait is called the phenotype. • PHenotype  PHysical • Ex) Bunny Eye Color • Genotype = Bb, Phenotype = brown eyes

7. Heredity • When a parent cell undergoes meiosis, the alleles separate into different sex cells. • The trait that gets passed on to the offspring (baby) depends on which pair of alleles the offspring happens to get. • Gregor Mendel pioneered the science of predicting which traits would be passed on from one generation to the next.

8. Punnett Squares • Mendel used punnett squares to help predict the genotypes of offspring. • A punnett square requires that you know the genotype of each parent. • Ex) Mother = BB (brown eyes), Father = bb (blue eyes) • The parent genotypes are put along the sides of a square cut into four segments. B B Mother b Father b

9. Punnett Squares • Then, the alleles from the mother are brought down and the alleles from the father are brought over. • Each of the four combinations equals a possible combination for the offspring. Mother B In this case, all of the offspring would be “Bb” and have brown eyes. B b B b B b brown eyes brown eyes Father b B b B b brown eyes brown eyes

10. Punnett Squares • What if the parents had different genotypes? • Ex) Mother = Bb (brown), Father = Bb (brown) In this case, ¾ of the offspring will have brown eyes and ¼ of the offspring will have blue eyes. Mother B b B B B b B brown eyes brown eyes Father B b b b b brown eyes blue eyes

11. Punnett Squares • Try this combination on your own. • Ex) Mother = bb (blue), Father = Bb (brown) In this case, ½ of the offspring will have brown eyes and ½ of the offspring will have blue eyes. Mother b b B b B b B brown eyes brown eyes Father b b b b b blue eyes blue eyes