PATTERNS OF INHERITANCE AND MEIOSIS
I. History
Name:________________________________
Date:__________________ Period:_________
A. Gregor Mendel – 1865 Father of Genetics published a scientific paper of his research results
(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
2. Mendel discovered that traits could disappear in one generation, only to reappear in
another generation, e.g. Parents (P1) with pure-breeding strains of red x white flowers
=> all red flowers (F1). Self-pollination of F1 => 3 red + 1 white (F2). This illustrates the
disappearance of the color white flower in F1 generation and reappearance in F2
generation. After many more crosses Mendel suggested that there must be two
heritable factors in each individual, and that these factors (alleles) 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 and Cc all produce colored 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 (9:3:3:1 ratio). 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 that are identical.
B. Meiosis occurs by duplication of chromosomes, but is followed by two reduction divisions.
Thus, one cell turns into four gametes that are not identical, but instead contain ½ the
genetic material as the original cell.
1. Prophase I: homologous chromosomes condense, undergo duplication (forming sister
chromatids) and crossing over, and centrioles move to the poles (opposite side of cell)
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 homologous chromosomes separate and are pulled to alternate
poles
4. Telophase I: chromosomes relax, 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 sister chromatids separate and are pulled to alternate poles
8. Telophase II: chromosomes relax, 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!
IV. Linkage (exceptions to Mendel's second law)
A. When genes are located near each other on a chromosomal, they do not assort independently.
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Deduction from nonindependent assortment
Utility for gene mapping
Huntington's chorea.
B. Sex linkage
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Color blindness and hemophilia