Genetics and Heredity

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

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

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

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

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

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.

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

Punnett Squares B B b B b B b b B b B b 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

Punnett Squares B b B B B b B B b b b b 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

Punnett Squares b b B b B b B b b b b b 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