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Human Inheritance. Chapter 14. Human Genetic Analysis. Inheritance patterns in humans are typically studied by tracking observable traits that crop up in families over many generations Data is organized in pedigree charts. Human Genetic Analysis. Pedigree chart symbols.
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Human Inheritance Chapter 14
Human Genetic Analysis • Inheritance patterns in humans are typically studied by tracking observable traits that crop up in families over many generations • Data is organized in pedigree charts
Human Genetic Analysis • Pedigree chart symbols
Human genetic analysis • Some easily observed human traits follow Mendelian inheritance patterns • Controlled by a single gene • Two alleles, one dominant and one recessive
Human Genetic Analysis • Mid-digital hair • Dominant condition (MM, Mm) • Presence of hair between the two top joints of your fingers • Even the slightest amount of hair indicates the dominant phenotype • Recessive condition (mm) • Complete absence of hair
Human Genetic Analysis • Tongue rolling • Dominant condition (TT, Tt) • Ability to roll one’s tongue • Recessive condition (tt) • Inability to roll the tongue
Human Genetic Analysis • Widow’s peak • Dominant condition (WW, Ww) • A distinct downward point in the frontal hairline • Recessive condition (ww) • A continuous hairline
Human Genetic Analysis • Earlobe attachment: • Dominant condition (EE, Ee) • Detached or free earlobes • Recessive condition (ee) • Earlobes attached directly to the head
Human Genetic Analysis • Hitchhiker’s thumb • Dominant condition (HH, Hh) • Cannot extend the thumbs backward to approximately 45° • Recessive condition (hh) • Can bend the thumbs at least 45°, if not further
Autosomal Inheritance Patterns • Dominant Patterns • Dominant alleles are expressed in both homozygotes and heterozygotes • The trait specified tends to appear in every generation
Autosomal Inheritance Patterns • Dominant Patterns • Examples • Achondroplasia • The allele interferes with formation of the embryonic skeleton • Adults average about 4’4” and have short limbs • Huntington’s disease • Nervous system slowly deteriorates • Involuntary muscle movements increase
Autosomal Inheritance Patterns • Recessive Patterns • Expressed only in homozygous people • Traits may skip generations • Heterozygotes are carriers of the allele, but do not express the trait
Autosomal Inheritance Patterns • Recessive Patterns • Examples • Albinism • Lack of melanin • Tay-Sachs disease • Malfunction of a lysosomal enzyme that breaks down gangliosides • Lipids accumulate to toxic levels in nerve cells
X-linked Inheritance Patterns • The X and Y chromosomes carry different genes • Recessive alleles on the X chromosome create a unique pattern of inheritance
X-linked Inheritance Patterns • More males affected • Each male receives one X and one Y chromosome • Two possible genotypes: XAY, XaY • Females receive two X chromosomes • Three possible genotypes: XAXA, XAXa , XaXa • Thus heterozygous males are affected while heterozygous females are not
X-linked Inheritance Patterns • Affected fathers cannot pass X-linked recessive alleles to a son • All children who inherit their father’s X chromosome are female XA XA Xa XAXa XAXa Daughters Y XAY XAY Sons
X-linked Inheritance Patterns • Affected fathers cannot pass X-linked recessive alleles to a son • Heterozygous females are the bridge between an affected male and his affected grandson Heterozygote daughter XA XA XA Xa Xa XAXa XAXa XA XAXA XAXa Affected father Y XAY XAY Y XAY XaY Affected grandson
X-linked Inheritance Patterns • Examples • Red-Green color blindness • Hemophilia A • Duchenne Muscular Dystrophy
X-linked Inheritance Patterns • X-linked dominant alleles that cause disorders are rarer because they tend to be lethal in male embryos • X-linked hypophosphatemic rickets • Rett syndrome • Aicardi syndrome
Heritable Changes in Chromosome Structure • Changes in chromosome structure • Are rare • Usually cause drastic health effects • Responsible for ~1/2 of miscarriages • Genetic disorders • Sometimes evolutionarily important • Occur spontaneously or induced by exposure to certain chemicals or radiation
Heritable Changes in Chromosome Structure • Changes in chromosome structure • Duplication • Happens during prophase I of meiosis • Crossing over occurs unequally between homologs • One chromosome will have a deleted segment • The other will have the duplication • Huntington’s
Heritable Changes in Chromosome Structure • Changes in chromosome structure • Deletion • The loss of some portion of a chromosome • Cause serious disorders and are often lethal in mammals • Cri-du-chat • Deletion in chromosome 5 • Causes mental impairment and an abnormally shaped larynx
Heritable Changes in Chromosome Structure • Changes in chromosome structure • Inversion • A segment of DNA is flips in the reverse direction • Usually no genes are lost • Causes infertility • Inverted segments cause homologs to mispair during meiosis
Heritable Changes in Chromosome Structure • Changes in chromosome structure • Translocation • A piece of one chromosome may break and attach • to a different chromosome or • to a different part of the same chromosome • Burkitt’s lymphoma: reciprocal translocation between chromosomes 8 and 14 • Can cause infertility (mis-pairing during meiosis)
Heritable Changes in Chromosome Structure • Chromosome changes in Evolution • Large-scale changes in chromosomes can lead to speciation • Most are usually lethal or cause genetic disorders and infertility • In a very few instances chromosome changes can be adaptive • Multiple globin chain genes possibly arose due to duplications • An individual homozygous for an inversion could become the founder of a new species
Heritable Changes in Chromosome Number • During meiosis homologs may fail to separate • Referred to as nondisjunction • Affects the chromosome number at fertilization • Normal gamete (n) + nondisjunction gamete (n + 1) = zygote (2n +1) • Trisomic zygote will have three of one type of chromsome and two of every other type • Normal gamete (n) + nondisjunction gamete (n - 1) = zygote (2n -1) • monosomic zygote will have one of one type of chromsome and two of every other type
Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Stepped Art Fig. 14.12, p. 212
Heritable Changes in Chromosome Number • Trisomy and monosomy • Trisomy (polyploid) is common in some plants, insects, and fish • Usually fatal in humans • Trisomy 21 individuals will survive infancy • Down syndrome
Heritable Changes in Chromosome Number • Changes in Sex Chromosome Number • Turner syndrome (XO) • Inherit an unstable Y from the dad • Female, short • XXX syndrome • Usually does not result in physical or medical problems • Klinefelter syndrome (XXY) • Overweight, tall, normal intelligence, estrogen > testosterone • XYY syndrome • Taller than average, mild mental impairment • NOT predisposed to a life of crime
Summary • Human genetic analysis • Pedigree charts • Autosomal inheritance patterns • Dominant and recessive • X-linked inheritance patterns • More common in males • Chromosome structure changes • Duplications • Deletions • Inversions • Translocations • Chromosome number changes • Trisomy • Sex chromosome numbers