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Genetics summary chapter 6 VU amsterdam
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- Vrije Universiteit Amsterdam (VU)
This summary covers complete chaper 6 of genetics at the Vrije Universiteit Amsterdam.
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Chapter 6- Genetic map: a diagram that shows the order of genes along a chromosome.
In eukaryotic species, each linear chromosome contains a very long segment of DNA along with many different proteins. A
chromosome contains many individual functional units – genes – that influence an organism’s traits. A typical chromosome
is expected to contain hundreds/few thousand genes.
- Synteny: refers to the situation in which two/more genes are located on the same chromosome. Genes that are
syntenic are physically linked to each other, because each chromosome contains a single, continuous, linear
molecule of DNA.
- Genetic linkage: the phenomenon in which genes that are close together on the same chromosome tend to be
transmitted as an unit. Therefore, genetic linkage has an influence on inheritance patterns.
Chromosomes are sometimes called linkage groups, because a chromosome contains a group of genes that are physically
linked together. In species that have been characterized genetically, the number of linkage groups equals the number of
chromosome types. For example, human somatic cells have 46 chromosomes, which are composed of 22 types of
autosomes plus one pair of sex chromosomes. Therefore, humans have 22 autosomal linkage groups and an X chromosome
linkage group, and human males also have the Y chromosome linkage group. In addition, the human mitochondrial genome
is another linkage group as well. Geneticists are often interested in the transmission of two or more characters in a genetic
cross.
- Two-factor cross: an experiment that follows the variants of two different characters in a cross.
- Three-factor cross: an experiment that follows the variants of three different characters (genes) in a cross.
The outcome of a two-factor or three-factor cross depends on whether or not the genes are linked to each other on the
same chromosome. Bateson and Punnet discovered two genes that did not assort independently. According to Mendel’s
law of independent assortment, a two-factor cross between individuals that are heterozygous for two genes should yield a
9:3:3:1 phenotypic ratio among the offspring. However, this was not the case when there was a cross of sweet peas
involving two different characters: flower color and pollen shape.
They began with crossing a true-breeding strain with purple flowers (PP) and long pollen (LL) to a true-breeding strain with
red flowers (pp) and round pollen (ll). This cross yielded an F1 generation of plants that all had purple flowers and long
pollen (PpLl). However, even though the F2 generation had 4 different phenotypic outcomes, it did not conform to a 9:3:3:1
ratio. Batson and Punnet found that the F2 generation had a much greater proportion of the two phenotypes found in the P
generation – purple flowers with long pollen and red flowers with round pollen. Therefore, they suggested that the
transmission of the two characters from the P generation to the F2 generation was somehow coupled; that is, the alleles
were not assorted in an independent manner. However, Bateson and Punnett did not realize that this coupling was due to
the linkage of the flower color gene and the pollen shape gene on the same chromosome.
Even when the alleles for different genes are linked on the same chromosome, the linkage can be altered during meiosis. In
diploid eukaryotic species, homologous chromosomes can exchange pieces with each other crossing-over. This event
mainly occurs during the prophase of meiosis I. The replicated chromosomes, sister-chromatids, form a structure known as
a bivalent. A bivalent is composed of two pairs of sister chromatids. In prophase of meiosis I, a sister-chromatid from one
pair may cross with a sister-chromatid from another homologous pair.
- Recombinant cells: when two pairs of homologous chromosomes exchange pieces with each other. The grouping
of the linked alleles has changed and the combinations of the alleles are different from the original chromosomes.
This results into recombinant cells.
If such haploid cells are gametes that participate in fertilization, the resulting offspring is called recombinant offspring.
These offspring can display combinations of traits that are different from those of either parent. In contrast, offspring that
have inherited chromosomes carrying the same combinations of alleles found in the chromosomes of their parents are
known as nonrecombinant offspring. When offspring inherit a combination of two or more alleles of traits that are different
from either of their parents, this event is known as genetic recombination. It commonly occurs in two ways:
1. When two/more genes are linked on the same chromosome, crossing-over during meiosis can result in genetic
recombination.
2. When two/more genes are on different chromosomes, the independent assortment of those chromosomes
during meiosis can result in genetic recombination.
The definition of recombinant offspring is: recombinant offspring are produced by crossing-over between two homologous
chromosomes during meiosis I in a parent, leading to a different combination of alleles along a chromosome compared to
that parent. Morgan provided evidence for the linkage of X-linked genes and proposed that crossing-over between X
chromosomes can occur. The first direct evidence that different genes are physically located on the same chromosome
came from the studies of Thomas Hunt and Morgan (Fly experiment with three characters). Morgan observed much higher
proportions of the combinations of traits found in the P generation. His explanation for this higher proportion of non-
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