Homework

Gender is determined by father, because he can give either the X or the Y chromosome.

It is estimated that from 10-20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure.

These chromosome abnormalities:

● arise from errors in meiosis, usally meiosis I; ● occur more often (90%) during egg formation than during sperm formation; ● become more frequent as a woman ages. ● Aneuploidy – the gain or loss of whole chromosomes – is the most common chromosome

abnormality. ● Aneuploidy is caused by nondisjunction, the failure of chromosomes to separate correctly

o homologues during meiosis I or o sister chromatids during meiosis II

● Zygotes missing one chromosome (“monosomy”) cannot develop to birth (except for females with a single X chromosome).

● Three of the same chromosome (“trisomy”) is also lethal except for chromosomes 13, 18, and 21 (trisomy 21 is the cause of Down syndrome).

● Three or more X chromosomes are viable because all but one of them are inactivated.

X-Inactivation Human females inherit two copies of every gene on the X chromosome, whereas males inherit only one (with 18 exceptions: the 9 pseudoautosomal genes and the 9 “housekeeping” genes found on the Y). But for the hundreds of other genes on the X, are males at a disadvantage in the amount of gene product their cells produce? The answer is no, because females have only a single active X chromosome in each cell. During interphase, chromosomes are too tenuous to be stained and seen by light microscopy. However, a dense, stainable structure, called a Barr body (after its discoverer) is seen in the interphase nuclei of female mammals. The Barr body is one of the X chromosomes. Its compact appearance reflects its inactivity. So, the cells of females have only one functioning copy of each X-linked gene – the same as males. X-inactivation occurs early in embryonic development. In a given cell, which of a female’s X chromosomes becomes inactivated and converted into a Barr body is a matter of chance (except in marsupials like the kangaroo, where it is always the father’s X chromosome that is inactivated). After inactivation has occurred, all the descendants of that cell will have the same chromosome inactivated. Thus X-inactivation creates clones with differing effective

 

 

gene content. An organism whose cells vary in effective gene content and hence in the expression of a trait, is called a genetic mosaic.

X-Chromosome Abnormalities:

As we saw above, people are sometimes found with abnormal numbers of X chromosomes. Unlike most cases of aneuploidy, which are lethal, the phenotypic effects of aneuploidy of the X chromosome are usually not severe. Examples:

● Turner’s syndrome X0 (2n= 45): females with but a single X chromosome. The phenotypic effect is mild because their cells have a single functioning X chromosome like those of XX females. Number of Barr bodies = zero.

● XXX, XXXX, XXXXX karyotypes: all females with mild phenotypic effects because in each cell all the extra X chromosomes are inactivated. Number of Barr bodies = number of X chromosomes minus one.

● Klinefelter’s syndrome (2n= 47): people with XXY or XXXY karyotypes are males (because of their Y chromosome). But again, the phenotypic effects of the extra X chromosomes are mild because, just as in females, the extra Xs are inactivated and converted into Barr bodies.

Genetic Disorders – Diseases resulting from abnormalities in human genes

• Mutation in a single gene – absence or alteration of a protein. Ex. Hemophilia • Chromosomal disorder – Duplication, loss, or breakage. Ex. Down Syndrome

There are many genetic diseases and disorders (See Hand Out). Some are:

• Down Syndrome • Sickle cell anemia • Hemophilia • Fanconi’s anemia • Cystic fibrosis

Many Gross Chromosomal abnormalities don’t cause death and are sex chromosome related. Karyotypes are used to look at chromosome shape and to ensure there are no abnormalities with the fetus. This is done by looking at the embryo’s amniotic fluid (amniocentesis).