Chromosomes and Human Inheritance Chapter 12

Strange Genes, Richly Tortured Minds Impacts, Issues Video Strange Genes, Richly Tortured Minds

Genes Units of information about heritable traits In eukaryotes, distributed among chromosomes Each has a particular locus Location on a chromosome

Homologous Chromosomes Homologous autosomes are identical in length, size, shape, and gene sequence Sex chromosomes are nonidentical but still homologous Homologous chromosomes interact, then segregate from one another during meiosis

Alleles Different molecular forms of a gene Arise through mutation Diploid cell has a pair of alleles at each locus Alleles on homologous chromosomes may be same or different

Karyotype Preparation

Karyotypes Karyotypes are used to identify defects in the chromosomes like: Missing chromosomes Multiple chromosomes

Karyotype Preparation

Autosomal Dominant Inheritance Trait typically appears in every generation

Autosomal Dominant Inheritance

Huntington Disorder Autosomal dominant allele Causes involuntary movements, nervous system deterioration, death Symptoms don’t usually show up until person is past age 30 People often pass allele on before they know they have it

Hutchinson-Gilford Progeria Mutation causes accelerated aging No evidence of it running in families Appears to be dominant Seems to arise as spontaneous mutation Usually causes death in early teens

Hutchinson-Gilford Progeria

The Y Chromosome Fewer than two dozen genes identified One is the master gene for male sex determination SRY gene (sex-determining region of Y) SRY present, testes form SRY absent, ovaries form

The X Chromosome Carries more than 2,300 genes Most genes deal with nonsexual traits Genes on X chromosome can be expressed in both males and females

X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father

Color Blindness

Color Blindness

Hemophilia

Fragile X Syndrome An X-linked recessive disorder Causes mental retardation Mutant allele for gene that specifies a protein required for brain development Allele has repeated segments of DNA

Chromosomal Mutations involve a change in the structure of a chromosome

Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses orientation of a segment within a chromosome Translocation moves a segment from one chromosome to another non homologous chromosome 23

Chromosomal Mutations 24

Duplications and translocations also tend to be harmful A diploid embryo that is homozygous for a large deletion is likely missing a number of essential genes; such a condition is generally lethal Duplications and translocations also tend to be harmful In inversions, the balance of genes is normal but phenotype may be influenced if the expression of genes is altered 25

Deletion Cru-Du-Chat Cru-Da-Chat Cry

Translocation A piece of one chromosome becomes attached to another nonhomologous chromosome Most are reciprocal Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22

Abnormal Chromosome Number Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do 28

NONDISJUNCTION Nondisjunction pairs of homologous chromosomes do not separate normally during meiosis as a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy 29

Meiosis I Nondisjunction Figure 12.13-1 Meiotic nondisjunction (step 1) 30

Meiosis I Nondisjunction Meiosis II Non- disjunction Figure 12.13-2 Meiotic nondisjunction (step 2) 31

Nondisjunction of homo- logous chromosomes in meiosis I (b) Meiosis II Non- disjunction Gametes Figure 12.13-3 Meiotic nondisjunction (step 3) n  1 n  1 n − 1 n − 1 n  1 n − 1 n n Number of chromosomes (a) Nondisjunction of homo- logous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II 32

only one copy of a particular chromosome Trisomic zygote Aneuploidy results from the fertilization of gametes in which nondisjunction occurred offspring with this condition have an abnormal number of a particular chromosome Monosomic zygote only one copy of a particular chromosome Trisomic zygote three copies of a particular chromosome 33

Aneuploidy Individuals have one extra or less chromosome (2n + 1 or 2n - 1) Major cause of human reproductive failure Most human miscarriages are aneuploids

Polyploidy Individuals have three or more of each type of chromosome (3n, 4n) Common in flowering plants Lethal for humans 99% die before birth Newborns die soon after birth

Down Syndrome Trisomy of chromosome 21 Mental impairment and a variety of additional defects Can be detected before birth Risk of Down syndrome increases dramatically in mothers over age 35

Down Syndrome

Patau Syndrome Trisomy 13 Serious eye, brain, circulatory defects as well as cleft palate 1:5000 live births

Edwards Syndrome Trisomy 18 Every organ is affected 1:10,000 births Life expectancy few months (full trisomy 18)

Turner Syndrome Inheritance of only one X (XO) 98% spontaneously aborted Survivors are short, infertile females No functional ovaries Secondary sexual traits reduced May be treated with hormones, surgery

Klinefelter Syndrome XXY condition Results mainly from nondisjunction in mother (67%) Phenotype is tall males Sterile or nearly so Feminized traits (sparse facial hair, somewhat enlarged breasts) Treated with testosterone injections

XYY Condition Taller than average males Most otherwise phenotypically normal Some mentally impaired Once thought to be predisposed to criminal behavior, but studies now discredit

Genetic Disorders and Genetic Abnormalities

Prenatal Diagnosis Amniocentesis

CVS - Chorionic villus sampling

Pedigree Analysis

Pedigree Analysis In humans, pedigree analysis is an important tool for studying inherited diseases Pedigree analysis uses family trees and information about affected individuals to: figure out the genetic basis of a disease or trait from its inheritance pattern predict the risk of disease in future offspring in a family (genetic counseling)

Pedigree Analysis I II Mating Normal Normal Female Male 1st born Affected Siblings

Pedigree’s How to read pedigrees Basic patterns of inheritance autosomal, recessive autosomal, dominant X-linked, recessive X-linked, dominant (very rare)

Autosomal Recessive Trait is rare in a pedigree Trait often skips generations (hidden in heterozygous carriers) Trait affects males and females equally Most common Autosomal Recessive disorders Cystic fibrosis Sickle cell anemia Phenylketonuria (PKU) Tay-Sachs disease

Autosomal Recessive

(parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Figure 11.14b Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) Figure 11.14b Pedigree analysis (part 2: attached earlobe) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? 60

Autosomal Dominant Trait is common in the pedigree Trait is found in every generation Affected individuals transmit the trait to approximately 1/2 of their children (regardless of sex)

Autosomal Dominant

(parents, aunts, and uncles) Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) Figure 11.14a Pedigree analysis (part 1: widow’s peak) WW or Ww ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? 63

X-Linked Recessive Trait is rare in pedigree Trait skips generations Affected fathers DO NOT pass to their sons, Males are more often affected than females

X-Linked Recessive

X-Linked Dominant Trait is common in pedigree Affected fathers pass to ALL of their daughters Males and females are equally likely to be affected X-linked dominant diseases are extremely unusual Often, they are lethal (before birth) in males and only seen in females ex. incontinentia pigmenti (skin lesions) ex. X-linked rickets (bone lesions)

X-Linked Dominant