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Mendelian Genetics Study Guide

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The branch of biology that studies heredity and traits is Genetics.
  1. Who is the “Father of Genetics”? Gregor Mendel
  2. Explain 3 reasons why Mendel chose pea plants to use in his experiments.
Can self-pollinate them, they had “either-or” traits, and short generation time.
  1. From his experiments with pea plants, Mendel concluded that each organism has 2 “factors” that control each trait and one factor comes from each parent. We now call these “factors” _____genes___ and the different versions of these “factors” are called _____ alleles_______.
  2. A ____ monohybrid____ cross is when only one trait is being examined (ex. Seed color). A _____dihybrid_______ cross is when two traits are being examined (ex. Seed color and height).
  3. The physical appearance of a trait (ex. tall or short) is called the ___phenotype____. The genetic makeup (Tt, TT, or tt) is the _____genotype_____.
  4. Explain the Law of Segregation.
Law of Segregation states that alleles separate, or segregate during gamete formation. This was shown in Mendel’s F2 generations when recessive traits reappeared.
  1. Explain the Law of Independent Assortment.
Law of Independent Assortment: genes for different traits independently assort during gamete formation and are passed down independently of each other. (note: this only applies when genes are located on different chromosomes.)
  1. If a parent’s genotype is AaBbcc, what are the possible allele combinations for its gametes?
ABc, Abc, aBc and abc.
8. A Punnett square is a tool used to predict the probability that a particular event will occur (ex. that offspring will have attached earlobes).
  1. The following trihybrid cross is made: AaBbcc x AabbCc. What is the probability that an offspring will be homozygous recessive for all three genes (aabbcc)?
probability would be ½ x ½ x ½ = 1/8 because the individual probability of getting heterozygous offspring from the parents is ½ for each gene.
Answer the following genetics questions and complete the corresponding Punnett Squares to show all possible genotypes that might result from the following parent combinations:
  1. In human beings brown eyes (B) are dominant to blue eyes (b). If a homozygous brown-eyed man marries a brown-eyed woman whose mother had blue eyes, what proportion of their children will be expected to have blue eyes? _______0%______
(Complete the square)
½ BB, brown
½ Bb, brown
BB x Bb B b
B
All offspring will get a B gene from
father, so all will have brown eyes.
  1. A brown-eyed woman (whose father was blue-eyed and whose mother was brown eyed) marries a blue eyed man. What is the genotype of the woman? ___Bb_______
Bb
bb
B b
What % of their children will have blue eyes? _____50%_____
  1. Two pure-breeding strains of pea are crossed. One strain is yellow and the other is green. All of the offspring in the F1 generation are yellow. A cross of the F1 individuals produces both yellow and green F2 pea plants. (Think back to Mendel’s first 2 crosses!)
  2. What is the genotype of the F1 pea plants? ____Yy______
  3. How many of the plants in the F2 generation would you expect to be yellow? ____75%___
  4. Two spotted rabbits are mated repeatedly. The offspring consisted of 27 spotted rabbits and 9 solid-colored rabbits.
  5. Is the solid color dominant or recessive? _____Solid is recessive; parents don’t show it__
  6. What are the genotypes of the two original parents? _____Must both be heterozygous__
  7. In humans, the ability to roll the tongue to make a trough is controlled by a dominant gene. A man and his wife can roll their tongues but their son cannot.
  8. What are the genotypes of the 3 individuals? Father_Aa_; Mother_ Aa _; Son_aa_
  9. In watermelons, the alleles for green color (G) and for short shape (L) are dominant over the alleles for striped color (g) and long shape (l). A plant with long, striped fruit (ggll) is crossed with a plant that is heterozygous for both traits (GgLl). Create a Punnett square for this cross.
What possible phenotypes will this cross produce? ______Punnett square should be 1x4; Phenotypes are ¼ green/short, ¼ green/long, ¼ striped/short, ¼ striped/long.
  1. In guinea pigs, the characteristics of black hair & short hair are dominant to white hair & long hair. A white and long-haired guinea pig is crossed with a heterozygous black, heterozygous short guinea pig. Answer the questions below and complete the Punnett square for the cross:
  2. What are the genotypes of the parents? bbhh & BbHh.
  3. What possible gametes can the first parent produce? _bh_ 2nd parent? BH, Bh, bH, bh
  4. What is the phenotypic ratio of the offspring?
¼ each of black/short, black/long, white/short, white/long
Punnett Square
BH Bh bH bh
BbHh,
Short black hair
Bbhh
Long black hair
bbHh
short white hair
bbhh
long white hair
bh
  1. In tomatoes, cut-leaf (L) is dominant to potato-leaf (l); purple stem (R) is dominant to green stem (r). Complete the Punnett square for a cross between two tomatoes that are both heterozygous for leaf style but both homozygous recessive for stem color. Llrr x Llrr
Lr lr
¼ LLrr
¼ Llrr
¼ Llrr
¼ llrr
Lr What are the phenotypes of the offspring?
_3/4 would be cut leaf with green stem;_
¼ would be potato-leaf with green stem.
lr
  1. In pea plants, yellow color (Y) is dominant to green color (y) and the gene for tall plants (T) is dominant to the gene for short plants (t). The following two individuals are crossed: TtYY x TtYy
  2. Create and complete a Punnett square for this cross. (Punnett square would have gametes TY and tY on one side and gametes TY, Ty, tY, and ty on the other side. All would be yellow; and ¾ would be tall and ¼ would be short.
  3. Would it be possible for any of the F1 plants to be green? No
  4. Explain your answer for B in terms of the alleles of the parents: See answer to “a” above.