recap

12.1 recap

Mendel showed that genetic determinants are particulate and do not “blend” when the two gametes combine. Mendel’s first law of inheritance states that two copies of a gene segregate during gamete formation. His second law states that genes assort independently during gamete formation. The frequencies with which different allele combinations will be expressed in offspring can be calculated with a Punnett square or using probability theory.

learning outcomes

You should be able to:

  • Compare and contrast hypotheses of inheritance.

  • Give examples of dominance and recessiveness.

  • Explain the law of segregation.

  • Predict the results of a monohybrid cross using a Punnett square.

  • Use a model to illustrate the law of independent assortment.

  • Apply probability concepts to predict the results of crosses.

  • Apply Mendel’s laws to analyze aspects of human pedigrees.

Question 1

What results seen in the F1 and F2 generations of Mendel’s mono-hybrid cross experiments refuted the blending theory of inheritance?

The F1 generation showed only one phenotype and it was a parental phenotype; blending predicted that the F1 would show an intermediate phenotype, not like either parent. The F2 generation also showed the phenotype of the other parent, not seen in the F1; blending predicted that the phenotype would disappear.

Question 2

Diagram the processes of meiosis that underlie the laws of segregation and independent assortment.

Question 3

In guinea pigs, black body color (B) is completely dominant over albino (b). For the crosses below, give the genotypes of the parents:

Parental phenotypes Black offspring Albino offspring Parental genotypes?
Black × albino 12 0
Albino × albino 0 12
Black × albino 5 7
Black × black 9 3

BB × bb; bb × bb; Bb × bb; Bb × Bb

Question 4

The pedigree below shows the inheritance of a rare mutant phenotype in humans, congenital cataracts (black symbols).

image

  1. Are cataracts inherited as an autosomal dominant trait? Autosomal recessive?

  2. Person #5 in the second generation mates with a man who does not have cataracts. Two of their four children, a boy and a girl, develop cataracts. What is the chance that their next child will be a girl with cataracts?

  1. Autosomal dominant
  2. ¼

Mendel’s laws of inheritance remain valid today; his discoveries laid the groundwork for all future studies of genetics. Inevitably, however, we have learned that things are more complicated than they seemed at first. Let’s take a look at some of these complications, beginning with the interactions between alleles.