Mendelian Inheritance I

21 October, 2002Text Chapter 14

Gregor Mendel’s experiments used pea plants as a model system. He examined the inheritance of characters like flower color and seed shape by mating plants and observing the offspring.

character: a heritable feature, like flower color.trait: a variant of a character, like purple or white flowers.

true-breeding: plants that, when self pollinated, produce offspring of the same variety

Mendel’s results refuted the blending hypothesis. He proposed a particulate theory of inheritance where characters are determined by genes (recipes for a character) that come in different versions (alleles).

Mendel followed heritable characters for three generations.

Diploid organisms have two copies of each gene. These copies can be the same or different. One copy was inherited from each parent.

If the two alleles differ, then one, the dominant allele determines the appearance of the organism.

During gamete formation, the two alleles segregate into gametes.

Alleles are different versions of a gene.

•Each parent has two alleles.•Gametes contain only one allele.

•Offspring have two alleles - one allele from each parent.•When both alleles are present, the dominant allele determines appearance.•Gametes contain only one allele.

•Offspring have two alleles - one allele from each parent.

•When both alleles are present, the dominant allele determines appearance.

•This leads to a 3:1 ratio of offspring.

Mendel’s Rules of Inheritance

•homozygous: a diploid organism that has two copies of the same allele for a given gene.

•heterozygous: a diploid organism that has two different alleles for a given gene.

•phenotype: an organism’s appearance.

•genotype: an organism’s genetic makeup, its collection of alleles.

Important terms:

We cannot be sure of the genotype of an individual with a dominant phenotype. That individual could be homozygous or heterozygous.

A testcross can reveal the genotype of the individual in question. A homozygous dominant individual will produce all dominant phenotype offspring in a testcross. A heterozygote will produce a 1:1 ratio of offspring (dominant to recessive phenotype).

Testcross

Independent Assortment

•Probability of an event is measured from 0 (impossible) to 1 (certain).

What is the probability of flipping a coin and getting tails?

1/2 or 0.5

What is the probability of rolling a 3 on a six-sided die?

1/6 or 0.167

When gametes are produced by a heterozygote, what is the probability of any given gamete having a recessive allele?

1/2 or 0.5

Mendelian inheritance follows the rules of probability (chance).

•Probability of an event that requires multiple steps is determined by multiplying the probabilities of the individual steps. This makes the probability get smaller.

What is the probability of flipping a coin twice, and getting tails both times?

(1/2)(1/2) = (1/4) or 0.25

When gametes are produced by an individual heterozygous at two loci (genes), what is the probability of any given gamete having recessive allele for both loci?

(1/2)(1/2) = (1/4) or 0.25

Mendelian inheritance follows the rules of probability (chance).continued...

•Probability of an event that has more than one way of occurring is determined by adding the probabilities of the individual ways. This makes the probability get larger.

What is the probability of flipping a coin twice, and getting one head and one tail?

Two ways: HT or TH, each(1/2)(1/2) = (1/4) or 0.250.25 +0.25 = 0.5 or 1/2

In the self-cross of a heterozygote (Aa), what is the probability of any individual offspring being heterozygous?

Two ways: aA or Aa, each(1/2)(1/2) = (1/4) or 0.250.25 +0.25 = 0.5 or 1/2

Mendelian inheritance follows the rules of probability (chance).continued...

In the self-cross of a heterozygote (Pp), what is the probability of any individual offspring being heterozygous?

Two ways: pP or Pp, each(1/2)(1/2) = (1/4) or 0.250.25 +0.25 = 0.5 or 1/2

Problem 1

In the three-factor cross where all three genes assort independently, PpYyRr x Ppyyrr, what is the probability of an offspring showing at least two recessive traits?

1. How many ways can this happen?

PPyyrr, Ppyyrr, ppYyrr, ppyyRr, ppyyrr

2. What is the probability of each way?

Ppyyrr (1/2)(1/2)(1/2) = 1/8 = 2/16PPyyrr (1/4)(1/2)(1/2) = 1/16ppYyrr (1/4)(1/2)(1/2) = 1/16ppyyRr (1/4)(1/2)(1/2) = 1/16ppyyrr (1/4)(1/2)(1/2) = 1/16

3. Add the probabilities of the ways. 2/16 + 1/16 + 1/16 + 1/16 + 1/16 = 6/16

Problem 2

How many unique gametes could be produced through independent assortment by an individual with the genotype Aa Bb CC Dd EE?

(2)(2)(1)(2)(1) = 8

Problem 3

You have been trying to train Great Northwestern rabbits to do a pole-vaulting routine whenever they hear a dog whistle. After much experimentation, you realize that the ability to perform pole-vaulting routines is determined by an autosomal gene in this species of rabbit. When you blow the whistle, the rabbits (depending on their genotype) either do the pole-vault or just ignore you and continue to chew thoughtfully on their carrots.

The results of a number of crosses with vaulters and non-vaulters are shown: Cross F1 progeny(i) vaulter x vaulter 12 vaulters, 0 non-vaulters(ii) vaulter x non-vaulter 8 vaulters, 7 non-vaulters(iii) non-vaulter x non-vaulter 4 vaulters, 13 non-vaulters

Which phenotype is dominant ? Explain your logic briefly (only one or two sentences needed!).

Problem 4

In rabbits, the homozygous CC is normal, Cc results in rabbits with deformed legs, and cc is lethal. For a gene for coat color, the genotype BB produces black, Bb brown, and bb a white coat. Give the phenotypic proportions of offspring from a cross of a deformed-leg, brown rabbit with a deformed-leg, white rabbit.

1. Write genotypes. CcBb x Ccbb

2. Determine gametes, probability of each. CB, Cb, cB, cb x Cb, cbProbabilities are 1/4, 1/4, 1/4, 1/4 x 1/2, 1/2

3. Fertilization: CCBb (1/8) brown, normal. CcBb (1/8) brown, deformed. CCbb (1/8) white, normal. Ccbb (1/8) white, deformed.CcBb (1/8) brown, deformed. ccBb (1/8) lethal. Ccbb (1/8) white, deformed. ccbb (1/8) lethal.

4. 1/3 white, deformed. 1/3 brown, deformed. 1/6 white, normal. 1/6 brown, normal.

Problem 5