Chapter 24: Patterns of Chromosome Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viewing the Chromosomes A karyotype is a display of chromosomes paired according to their size, location of the centromere, and staining patterns. A karyotype reveals abnormalities in chromosome number or structure. Humans have 23 pairs of chromosomes; 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY. All cells in the body except red blood cells have a nucleus and can the karyotyped. In adults, white blood cells are often selected for karyotyping. In fetuses, if abnormalities are suspected, cells can be obtained by either amniocentesis or chorionic villi sampling.

Normal male karyotype 2 copies of 22 autosomal chromosomes Fig. 24.1 2 copies of 22 autosomal chromosomes 2 copies of 1 sex chromosome Here is a karyotype of a normal male with 46 chromosomes.

Down syndrome karyotype Fig. 24.1 (Trisomy 21) This karyotype shows an individual (male) with Down syndrome having an extra chromosome 21. Total of 47 chromosomes

Changes in Chromosome Number Nondisjunction occurs when: 1.) both members of a homologous pair go into the same daughter cell or 2.) when sister chromatids fail to separate and both daughter chromosomes go into the same gamete. The result is a trisomy or a monosomy. Trisomy is a condition in which there is an extra chromosome; monosomy is a condition in which there is one chromosome missing.

Nondisjunction in meiosis I Fig. 24.2 Nondisjunction can occur during meiosis I. The result is abnormal eggs with either one too many or one too few chromosomes. Fertilization with a normal sperm results in an abnormal number of chromosomes in the zygote. Note that two of the eggs could potentially have the correct number of chromosomes. trisomy monosomy

Nondisjunction in meiosis II Fig. 24.2 Nondisjunction can occur during meiosis II if the sister chromatids separate but the resulting chromosomes go into the same daughter cell. The egg has 24 chromosomes; after fertilization with normal sperm, the zygote would have 47 instead of the normal 46 chromosomes. Alternatively, the egg could have 22 chromosomes, so after fertilization, the zygote has 45 rather than 46 chromosomes. monosomy trisomy

The Barr body is an inactive X chromosome and is seen whenever more than one X chromosome is present (i.e., XX, XXY, XXX). Cells of females function with a single chromosome just as those of males do.

Down Syndrome Down syndrome is caused by trisomy 21, a result of nondisjunction. Nondisjunction risk increases after age 40.

Changes in Sex Chromosome Number The presence/absence of a Y chromosome determines maleness. An abnormal number of sex chromosomes is the result of inheriting too many or too few X or Y chromosomes.

Turner syndrome: females with one X chromosome; XO. Klinefelter syndrome: males that have two or more X chromosomes and a Y chromosome. A Poly-X female has more than two X chromosomes and extra Barr bodies. Jacobs syndrome males are XYY. XO signifies the lack of a second X chromosome. Although their reproductive organs are underdeveloped, some females with Turner syndrome have given birth following in vitro fertilization using donor eggs.

Changes in Chromosome Structure A mutation is a permanent genetic change. Radiation, organic chemicals, or even viruses may cause chromosomes to break, leading to mutations. A change in chromosome structure is a chromosome mutation.

Chromosomal mutations include: 1.) deletion 2.) duplication 3.) inversion 4.) translocation

Deletions Deletions occur when part of the chromosome is lost. An individual with a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome may result. Fig. 24.5

Duplications Duplication: a chromosome segment is repeated in the same chromosome. Fig. 24.6 Inverted means that the duplicated segment joins in the direction opposite from normal.

Translocation Translocation is exchange of chromosomal segments between two, nonhomologous chromosomes (Ex: part of chromosome 2 is swapped with part of chromosome 7). Fig. 24.7 A person who has both of the involved chromosomes has the normal amount of genetic material and is healthy, unless the chromosome exchange breaks an allele into two pieces. The person who inherits only one of the translocated chromosomes will no doubt have only one copy of certain alleles and three copies of certain other alleles. A genetic counselor begins to suspect a translocation has occurred when spontaneous abortions are commonplace and family members suffer from various syndromes.

Inversion Inversion involves a segment of a chromosome being turned 180 degrees; the reverse sequence of alleles can alter gene activity. See Fig. 24.8 A B C D E F NORMAL A B C D E F INVERSION

Sex-Linked Traits Traits controlled by genes on the X or Y chromosomes are called sex-linked (i.e., on sex chromosomes). An allele on the X chromosome that is in the region where the Y chromosome has no alleles will express even if recessive; it is termed X-linked. A female would have to have two recessive genes to express the trait; a male would only need one. Usually a sex-linked (X-linked) disorder is recessive; a female would have to inherit the disorder for both parents to express the disorder, while the male inherits it only from his mother who passes her X chromosome on to him.

XX = female XY = male X Y X XX XY Y Chromosome determines sex of offspring

Comparison of the sex chromosomes Male Female X Y X X Alternate alleles available. Alternate alleles available. No alternate alleles available.

X-Linked Alleles The key for an X-linked problem shows the allele attached to the X as in: XB = normal vision Xb = color blindness. Females with the genotype XBXb are carriers because they appear to be normal but each son has a 50% chance of being color blind depending on which allele the son receives. XbXb and XbY are both colorblind. The inheritance from their father cannot offset the recessive allele because the Y chromosome does not carry X-linked alleles.

Cross involving an X-linked allele A woman is not color blind but her father was color blind. She has a child with a man that is not color blind. Could her children be color blind? Woman XB Xb Man XB Y Y XB 1 Color blind The male parent is normal, but the female parent is a carrier; an allele for colorblindness is located on one of her X chromosomes. Therefore, each son stands a 50% chance of being color blind. The daughters will appear normal, but each one stands a 50% chance of being a carrier. XB XB Y XB XB Not Color blind 3 Xb XB Xb Xb Y

X-Linked Disorders In pedigree charts that show the inheritance pattern for X-linked recessive disorders, more males than females have the trait. X-linked recessive disorders include red-green color blindness, muscular dystrophy, and hemophilia.

X-linked recessive pedigree chart (Color Blindness) Pedigree charts of X-linked recessive disorders show: Males more than females are affected. An affected son can have parents who have the normal phenotype. For a female to have the characteristic, her father must also have it, and her mother either has it or is a carrier. The characteristic often skips a generation from the grandfather to the grandson. If a woman has the characteristic, all of her sons will have it.

Color Blindness Three types of cones are in the retina detecting red, green, or blue. Genes for blue cones are autosomal; those for red and green cones are on the X chromosome. Males are much more likely to have red-green color blindness than females. About 8% of Caucasian men have red-green color blindness.

Other X-linked Disorders Muscular dystrophy is characterized by the wasting of muscles. Hemophilia blood does not clot normally. Fragile X syndrome one of the most common forms of mental retardation. -CGG Repeats