Use a useful tool, the Punnett square, to predict the probabilities of offspring gender and genotypes and phenotypes of different matings based on parental genetic makeup.
Part I: Mendelian Genetics
In this assignment, you will use a useful tool, the Punnett square, to predict the probabilities of offspring gender and genotypes and phenotypes of different matings based on parental genetic makeup. Please answer all of the bulleted questions and tasks as you read through this assignment and submit them as a Word document to the assignment drop box titled “Punnett Squares Assignment.”
Each person has two copies of each chromosome, one from each parent. Your genome is exactly one-half your father’s genome and one-half your mother’s genome. After sperm and egg meet, the baby carries both copies of each gene in every cell for the rest of his or her life…except when eggs or sperm are produced. The eggs or sperm receive only one copy of each chromosome and the cycle starts all over again. Exactly which half will the baby get? That is the random part.
Sexual reproduction relies on chance to determine what type of offspring will result. A couple anxiously awaits a boy or girl and a dog breeder anxiously awaits the colors and markings of the puppies to be born. Although there is a random element involved, offspring from a mating will follow mathematical laws of probability based on the genetic makeup of the mother and father.
Watch this video to learn about Punnett squares. Please recall that dominant alleles mask recessive alleles and each baby has two copies of each gene, one from each parent.
Complete a Punnett square for the cross Bb x bb, where B is brown eyes and b is blue eyes.
What percentage of offpring will be BB? Bb? bb?
What percentage will have blue eyes and what percentage will have brown eyes?
Part II: Sex Determination
A male carries an X chromosome and a Y chromosome. A female carries two X chromosomes.
After meiosis, sperm and egg are produced. Sperm have a 50% chance of carrying and X chromosome and a 50% chance of carrying a Y chromosome. Eggs have a 100% chance of carrying an X chromosome, because females ONLY carry X chromosomes.
(X) (X) (Y) (X) (X) (X)
(Y) (Y) (X) (X) (X) (X)
We can represent four potential scenarios if these two parents produce offspring neatly in a Punnett square. The possible sperm are placed above the top of the Punnett Square (circled in blue) and the possible eggs are placed along the left side beside the Punnett Square (circled in red). Each box is then filled in with the letter of each column and row.
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