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Human Inheritance Chapter 14 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011. Human Chromosomes Human body cells have 23 pairs of homologous chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes Autosomes and Sex Chromosomes Paired autosomes • Same in length and shape • Have the same centromere location • Carry the same genes along their length Sex chromosomes • Identical in human females (XX) • Nonidentical in human males (XY) Sex Determination SRY gene on the Y chromosome • Basis of male sex determination • Expression initiates synthesis of testosterone • Hormone causes embryo to develop into a male If an embryo has no Y chromosome (no SRY gene), it develops into a female Sex Determination Pattern Sexual Development in Human Embryos At seven weeks, appearance of “uncommitted” duct system of embryo Y chromosome present Y chromosome absent At seven weeks, appearance of structures that will give rise to external genitalia Y chromosome present Y chromosome absent 10 weeks 10 weeks ovary penis vaginal opening uterus penis testis vagina birth approaching Fig. 11.2, p.170 Autosomal Inheritance Patterns Some alleles on autosomes are inherited in simple Mendelian patterns associated with specific phenotypes Certain mutated forms of alleles give rise to genetic abnormalities or genetic disorders Table 14.1 in Text for Examples Autosomal Dominant Inheritance Autosomal Recessive Inheritance Key Concepts: AUTOSOMAL INHERITANCE Many genes on autosomes are expressed in Mendelian patterns of simple dominance X-Linked Inheritance Patterns Certain dominant and recessive alleles on the X chromosome are inherited in Mendelian patterns Mutated alleles on the X chromosome contribute to more than 300 known genetic disorders Males can’t transmit recessive X-linked alleles to sons (son receives X chromosome from mother) X-Linked Inheritance Patterns Hemophilia A X-linked recessive Interfere with blood clotting. Affect 1:7500 people. Pedigree chart showing the pattern of inheritance of a trait through generations in a family Green Carriers Red Diseased Fig. 11.8, p.174 Color Blindness Key Concepts: SEX-LINKED INHERITANCE Some traits are affected by genes on the X chromosome Inheritance patterns of such traits differ in males and females Heritable Changes in Chromosome Number Chromosome number of a parental cell can change permanently Often caused by nondisjunction • Failure of one or more pairs of duplicated chromosomes to separate during meiosis Nondisjunction in Meiosis n+1 n+1 n-1 n-1 chromosome alignments at metaphase I NONDISJUNCTION alignments at AT ANAPHASE I metaphase II anaphase II CHROMOSOME NUMBER IN GAMETES Stepped Art Fig. 11-13, p.178 Aneuploidy Aneuploidy Cells with too many or too few copies of a particular chromosome • In humans, the most common aneuploidy (trisomy 21) causes Down syndrome Most other human autosomal aneuploids die before birth Aneuploidy: Trisomy 21 (Down Syndrome) Fig. 11.14, p.179 Polyploid Polyploid having 3 or more of each type of chromosome characteristic of the species • Example: XXY (1:500 males) • Klinefelter syndrome • Example: XYY Polyploid XXY (1:500 males) • Klinefelter syndrome Characteristics • Overweight, tall, within normal range of intelligence • Make more estrogen and less testosterone than normal males • Small testes and prostate glands, low sperm counts, sparse facial and body hair, high-pitched voices, and enlarged breast • Testosterone injections during puberty can reverse these traits XYY • Normal OR May be taller with mild mental impairment Animation: Autosomal dominant inheritance Animation: Autosomal recessive inheritance Animation: Human sex determination Animation: Pedigree diagrams Animation: X-linked inheritance