Patterns of Inheritance and Meiosis



I. History

A. Blending inheritance - Darwin and others. Cross large with small and get intermediate

B. But, variation would be halved every generation. How can you explain two dark-eyed people giving birth to a blue-eyed child? or the spread of a rare form?

C. Gregor Mendel - 1865 (Darwin had a copy of Mendel's paper, but never opened it)

1. Investigated inheritance using pea plants. Recorded 7 traits, including color and shape of seeds, flower color, pod color and shape, height and position of flowers (overhead)

2. Mendel discovered that traits could disappear in one generation, only to reappear in another generation, e.g. pure-breeding strains of red x white flowers => all red flowers (F1). Self-pollination of F1 ~> 3 red + 1 white. After many more crosses Mendel suggested that there must be two heritable factors in each individual, and that these factors segregate at random into gametes prior to mating. This conclusion is known as

Mendel's first law: the Law of Segregation.

3. We can predict Mendel's monohybrid results if CC, Cc and cC all produce red flowers while cc produces white flowers. Then CC (red) x cc (white) = Cc (red). We say that C and c are alternate alleles at the flower color locus. In this example, red flower color is dominant, i.e. the heterozygote genotype produces the same phenotype as one of the homozygote genotypes. If one phenotype is dominant, then the alternate phenotype is termed recessive. We see segregation of phenotypes in the F2 generation when two heterozygotes are mated together.

4. A simple way to predict the relative frequencies of each phenotype is to use a Punnett square, in which rows represent all possible alleles carried by eggs and columns represent all possible alleles carried by sperm. Then, each cell represents the genotype of the resulting zygote. Note, that if C and c are equally common in eggs and sperm, then there will be 3 red genotypes and 1 white genotype, or a 3:1 ratio.

5. Mendel also collected data on crosses between strains which differed in two traits. For example, he crossed a plant with round yellow seeds with a plant having wrinkled green seeds. The resulting F1 dihybrid cross produced approximately 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green plant. These ratios are predicted from a Punnett square in which each gamete carries an allele from two loci. One locus is for seed shape and the other is for seed color. R is dominant to r and causes round seeds while G is dominant to g and causes yellow seeds. The tendency for two traits to be inherited independently is known as

Mendel's second law: the Law of Independent Assortment.

II. Meiosis causes segregation and independent assortment

A. Mitosis occurs by duplication of chromosomes, followed by one reduction division, thus one cell turns into two cells

B. Meiosis occurs by duplication of chromosomes, but is followed by two reduction divisions. Thus, one cell turns into four gametes

1. Prophase I: chromosomes condense, undergo duplication and crossing over, and centrioles move to the poles

2. Metaphase I: microtubules connect from the centrioles to the centromeres, and the paired chromosomes line up along the equator of the cell

3. Anaphase I: paired chromosomes separate and are pulled to alternate poles

4. Telophase I: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together

5. Prophase II: chromosomes condense and centrioles move to the poles

6. Metaphase II: microtubules connect from the centrioles to the centromeres, and the paired chromatids line up along the equator of the cell

7. Anaphase II: paired chromatids separate and are pulled to alternate poles

8. Telophase II: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together

III. Consequences of meiosis

A. Gamete formation. Note that meiosis in males (the sex with the smaller gamete) produces four functional gametes, while meiosis in females (the sex with the larger gamete) produces one functional gamete and three polar bodies, which disintegrate. This difference in gamete size is called anisogamy. Some plants are isogamous and do not produce polar bodies.

B. Meiosis also differs from mitosis because crossing over can occur during Prophase I. Crossing over is also known as recombination. Recombination together with independent assortment of chromosomes into gametes is responsible for Mendel's second law: the Law of Independent Assortment.

C. Maintains variation: recombination + independent assortment of chromosomes

1. Due to chromosomal segregation -> produces 2^n types of gametes. With 23 pairs of chromosomes, each human produces 8 million gamete types without crossing over
2. This produces 70 trillion zygote combinations
3. And, each meiosis typically has 30 crossing-over events!

Question: Then why do siblings look alike?

Answer: because the 8 million gametes from each parent are not independent between siblings. On average, siblings share half their genes - the probability of inheriting an identical allele from mom is 1/4 and the chance of inheriting it from dad is 1/4, so the combined probability is 1/2. Nevertheless, every full-sib is likely to be different.

IV. Linkage (exceptions to Mendel's second law)

A. When genes are located near each other on a chromosomal, they do not assort independently.

1. Deduction from nonindependent assortment
2. Utility for gene mapping
3. Huntington's chorea

B. Sex linkage

1. White in Drosophila
2. Color blindness and hemophilia