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Genetics summary chapter 5 VU Amsterdam
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Chapter 5 – Non-mendelian inheritanceGenes that follow a Mendelian inheritance pattern conform to 4 rules:
1. The expression of the genes in the offspring directly influence their traits.
2. Except in rare cases of rare mutations, the genes are passed unaltered from generation to generation.
3. The genes obey Mendel’s law of segregation.
4. For crosses involving two or more genes, the genes obey Mendel’s law of independent assortment.
Most genes in eukaryotic species follow a Mendelian inheritance pattern. However, many genes do not. We begin by
considering genes that have a maternal effect. For these genes, the genotype of the mother directly determines the
phenotype of her offspring. The genotypes of the father and of the offspring themselves do not affect the phenotype of the
offspring. Therefore, you cannot use a Punnet square to predict the phenotype of the offspring. We will see that maternal
inheritance is explained by the accumulation of gene products that the mother provides to her developing oocytes
(immature eggs).
The first example of maternal effect gene was studies by looking at a snail (Lymnaea peregra). In this species, the shell and
internal organs can be arranged in either right-handed (dextral) or left-handed (sinistral) direction. The dextral orientation is
more common and dominant. Boycott began with two different true-breeding strains of snails with either a dextral or
sinistral morphology.
- F1 generation: when a dextral female (DD) was crossed with a sinistral male (dd), all F1 offspring were dextral.
However, in the reciprocal cross, where a sinistral female (dd) was crossed to a dextral male (DD), all F1 offspring
were sinistral. Alfred Sturtevant proposed the idea that snail coiling is due to a maternal effect gene that exists as
a dextral (D) or sinistral (d) allele.
- F2 generation: the genotype of the F1 generation is expected to be heterozygous (Dd). When these F1 individuals
were crossed with each other, the predicted genotype ratio for the F2 generation was 1 DD: 2 Dd: 1 dd. Because
the D allele is dominant to the d allele, a 3:1 phenotypic ratio of dextral to sinistral snails could be observed,
according to a Mendelian inheritance pattern. However, the F2 generation consisted of all dextral snails. This is
due to maternal effects. The phenotype of the offspring depended on the genotype of the mother. The F1
mothers were Dd. The D allele in the mothers is dominant to the d allele and caused the offspring to be dextral,
even when the genotype of the offspring was dd!
- F3 generation: when the members of the F2 generation were crosses, the F3 generation exhibited a 3:1 ratio of
dextral to sinistral snails. This ratio corresponds to the ratio of the F2 females (the mothers of the F3 generation).
The ratio of F2 females was 1: DD, 2: Dd, 1: dd. The DD and Dd females produced dextral offspring, whereas the
dd females produced sinistral offspring. This explains the 3:1 ratio of dextral and sinistral offspring in the F3
generation.
At the molecular level and cellular levels, the non-Mendelian inheritance pattern of maternal effect genes can be explained
by the process of oogenesis in female animals. As an animal oocyte matures, many surrounding maternal cells called the
nurse cells provide the oocyte with nutrients and other materials. When a female is heterozygous (Dd), the haploid oocyte
may receive the D or d allele, but not both. The surrounding nurse cells, however, produce both D and d gene products
(mRNA and proteins). These gene products are then transported in the oocyte. These gene products exist for a significant
time after the egg has been fertilized and embryonic development has begun. In this way, the gene products of the nurse
cells, which reflect the genotype of the mother, influence the early development stages of the embryo.
- A female snail that is DD transmits the D gene product to the oocyte. During the early stages of embryonic
development, this gene product causes the embryo cleavage to occur in a way that produces a right-handed body
plan (dextral)
- A heterozygous female transmits both D and d gene products. Because the D allele is dominant, the maternal
effect also causes a right-handed body plan (dextral).
- A dd mother contributes only the d gene product that produces a left-handed body plan (sinistral), even if the egg
is fertilized by a sperm carrying a D allele. The sperm’s genotype is irrelevant, because the expression of the
sperm’s gene will occur too late.
Maternal effect genes encode proteins that are important in the early steps of embryogenesis. The accumulation of
maternal gene products in the oocyte allows embryogenesis to proceed quickly after fertilization. Maternal effect genes
often play a role in cell division, cleavage pattern, and body axis orientation. Therefore, defective alleles in maternal effect
genes tend to have a dramatic effect on the phenotype of the offspring, altering major features of morphology, often with
dire consequences.
- Epigenetics: the study of modifications that occur to a gene or chromosome that alters gene expression, but is
not permanent over the course of many generations.
Epigenetic changes alter the expression of particular genes in a way that may be fixed during an individual’s life. Therefore,
epigenetic changes can permanently affect the phenotype of the individual. However, epigenetic modifications are not
permanent over the course of many generations and they do not change the actual DNA sequence. For example, a gene
, Chapter 5 – Non-mendelian inheritance
may undergo an epigenetic change that inactivates it for a lifetime of an individual. However, when this individual makes
gametes, the gene may become activated and remain active during the lifetime of an offspring who inherited the gene.
- Dosage compensation: the phenomenon in which the level of expression of many genes on the sex chromosomes
(such as the X chromosome) is similar in both sexes even though females and males have different complement of
sex chromosomes. The difference in gene dosage – two copies in females versus one copy in males – is being
compensated for at the level of the gene expression. One copy of the allele in the female is not equivalent to one
copy of the allele in male. Instead, two copies of the allele in the female produce a phenotype that is similar to
that produced by one copy in the male.
a. Female mammals equalize the expression of the X-linked genes by turning off one of their two X-chromosomes
X-chromosome inactivation (XCI).
b. In Drosophilia, the level of expression on the X-chromosome in males is doubled.
c. In C.elegans, the XX animal is a hermaphrodite that produces both sperm and egg cells, and in animal carrying a
single X-chromosome is a male that produces only sperm. The level of expression of genes on the X-chromosome
in hermaphrodites is decreased to 50% of the level occurring on the X chromosome in males.
d. In birds, the Z chromosome is a large chromosome which contains almost all of the known sex-linked genes. The
W chromosome is generally much smaller and contains a higher proportion of repeat sequence DNA that does not
encode genes. Male birds are ZZ and females are ZW. Some Z-linked genes may be dosage-compensated, but
many of them are not.
Mary Lyon proposed that dosage compensation in mammals occurs by the inactivation of a single X-chromosome in
females.
- Barr body: highly condensed structure (X chromosome) in the interphase nuclei of somatic cells in females and
not in males.
- Clone: all cells within a clone are derived from a single cell.
A calico cat (picture on page 111) shows a female cat that is heterozygous for an X-linked gene that can occur as an orange
or a black allele. The orange and black patches are randomly distributed in different female individuals. The calico pattern
does not occur in male cats, but similar kinds of mosaic patterns have been identified in the female mouse. The pattern of
black and orange fur on the cat is due to random X-chromosome inactivation during embryonic development. The orange
patches of fur are due to the inactivation of the X-chromosome that carries a black allele; the black patches are due to the
inactivation of the X-chromosome that carries the orange allele. Lyon suggested that both the Barr body and the calico
pattern are the result of the X-chromosome inactivation (XCI) in the cells of the female mammals. The proposed mechanism
of the XCI is known as the Lyon hypothesis (see page 111 5.4). According to the Lyon hypothesis, each somatic cell in
female mammals expressed the genes on one of the X chromosomes, but not both. If an adult female is heterozygous for an
X-linked gene, only one of the two alleles will be expressed in any particular somatic cell.
During XCI, the chromosomal DNA of the inactivated X chromosome becomes highly compacted into a Barr body, so most
genes on that chromosome cannot be expressed. When cell division occurs and the inactivated X chromosome is replicated,
both copies remain highly compacted and inactive, so the inactive X-chromosome is passed along to the future somatic
cells.
Research has shown that mammalian somatic cells are able to count the X chromosomes they contain and allow only one of
them to stay active. This was determined due to an experiment that looked at people who were born with normal or
abnormal numbers of sex chromosomes. In normal females, two X chromosomes are counted and one is inactivated,
whereas in males, one X chromosome is counted and is not inactivated. If the number of X chromosomes exceeds two, as in
triple X syndrome, additional X chromosomes are converted into Barr bodies, so in this case it would be 2 inactivated X
chromosomes. A short region on the X chromosome called the X-inactivation center (Xic) is known to play an important
role in XCI. If one of the two X chromosomes in a female is missing its Xic due to a chromosome mutation, a cell counts only
one Xic and XCI does not occur. Having two active X chromosomes is a lethal condition for a human female embryo. The
process of XCI can be divided into three phases:
1. Nucleation: it occurs during embryonic development, one of the X chromosomes remains active and the other
one is chosen to be inactive.
2. Spreading: the chosen X chromosome is inactivated. The spreading phase is so named because inactivation begins
at the Xic and spreads in both directions along the X chromosome.
3. Maintenance: the inactivated X chromosome is maintained as a Barr body during future cell divisions. When a cell
divides, the Barr body is replicated, and both copies remain compacted. This maintenance phase continues from
embryonic stage through adulthood.
Some genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals. These genes are
said to escape the effects of XCI. In humans, up to a quarter of X-linked genes may occur in clusters. Among these are the
pseudoautosomal genes found on the X and Y chromosomes in the regions of homology. Dosage compensation is not
necessary for X-linked pseudoautosomal genes because they are located on both X and Y chromosomes. The expression of
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