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Summary Chapter 5 - Non-Mendelian Inheritance
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Chapter 5 of Genetics by Brooker.
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Chapter 5 - Non mendelian inheritanceGenes that follow a mendelian inheritance pattern conform four rules:
1. The expression of the genes in the offspring directly influences their traits.
2. Except in the case 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.
5.1 Maternal effect
For genes with a maternal effect the genotype of the mother directly determines the
phenotype of the offspring. For maternal effect genes, the genotypes of the father and the
offspring themselves do not affect the phenotype of the offspring. The inheritance is
explained by the accumulation of gene products that the mother provides to her developing
oocytes.
The genotype of the mother determines the phenotype of the offspring for maternal
effect genes.
Female gametes receive gene products from the mother that affect early
developmental stages of the embryo. Nurse cells provide the oocyte with nutrients and
other materials. The female has Dd as genotype. Depending on the meiosis, the haploid
oocyte may receive D or d, but not both. The surrounding nurse cells do produce products of
both D and d. These products are transported into the oocyte. These gene products persist
for a significant time after the egg has been fertilized and embryonic development has
begun. The gene products of the nurse cells, which reflect the genotype for the mother,
influence the early developmental stages of the embryo.
Maternal genes often play a role in cell division, cleavage pattern and body axis orientation.
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.
5.2 Epigenetics: Dosage compensation
Dosage compensation results in similar levels of gene expression between the sexes.
Dosage compensation refers to the phenomenon in which the level of expression of many
genes on the sex chromosomes is similar in both sexes even though males and females
have a different complement of sex chromosomes. Female mammals equalize the
expression of X-linked genes by turning off one of their two X-chromosomes.
Dosage compensation occurs in female mammals by the inactivation of one X
chromosome. The inactive X chromosome turns into a Barr body, which is a highly
condensed X chromosome. The mechanism of XCI, known as the Lyon hypothesis, is
schematically illustrated in figure 5.4.
All cells within a clone were derived from a single cell.
Mammals maintain one active X chromosome in their somatic cells. In normal females,
two X-chromosomes are counted and one is inactivated, whereas in males, one X-
chromosome is counted and none of them are inactivated.
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