Human Inheritance. Tracking Traits in Humans. Autosomal Inheritance. Sex-Linked Inheritance. Chromosome Changes. Genetic Testing

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1 Tracking Traits in Humans Autosomal Inheritance Sex-Linked Inheritance Chromosome Changes Genetic Testing Human Inheritance

2 How Do We Study Inheritance Patterns In Humans? Ø Geneticists often use historical records Ø Make charts (pedigrees of genetic connections) Ø Allow geneticists to determine the probability that a trait will recur in future generations Ø Determine whether a trait is associated with dominant or recessive alleles Ø Determine whether the alleles are on autosomes or sex chromosomes

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4 Patterns of Genetic Disorders Ø Autosomal dominant inheritance pattern Ø Autosomal recessive inheritance pattern Ø X-linked recessive inheritance pattern Ø X-linked dominant inheritance pattern Ø Changes in chromosome number Ø Changes in chromosome structure

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6 How Do We Know When A Trait Is Affected By An Allele On An Autosome?

7 How Do We Know When A Trait Is Affected By An Allele On An X Chromosome? X-linked recessive pattern Ø An allele is inherited on the X chromosome Ø Most are recessive, because X-linked dominant alleles tend to be lethal in male embryos

8 X-Linked Traits Ø X-linked recessive disorders tend to appear in men more often than in women Ø Men (XY) have only one X chromosome Ø Women have two X chromosomes (XX) Ø Men can transmit an X-linked allele to daughters, but not to sons only a woman can pass an X-linked allele to a son

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12 How Does Chromosome Structure Change? Chromosome structural changes Ø Include duplications, deletions, inversions, and translocations Ø Induced by: exposure to chemicals or radiation, faulty crossing over, transposable elements Ø Tend to result in genetic disorders

13 Deletion Loss of a part of a chromosome Duchenne muscular dystrophy/ deletion in the X chromosome Cri-du-chat/ deletion in the short arm of chromosome 5

14 Duplication Repeated section of a chromosome Expansion mutations Causes genetic abnormalities or disorders Huntington s disease

15 Inversion Part of a chromosome becomes oriented in the reverse direction May not affect carrier s health May affect fertility

16 Translocation two chromosomes exchange broken parts Most translocations are reciprocal (between nonhomologous chromosomes) Can affect fertility

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19 What Are The Effects of Chromosome Number Changes in Humans? Occasionally abnormal events can occur Ø Happen mostly during meiosis Ø New individuals end up with the wrong chromosome number Ø Consequences range from minor to lethal

20 Nondisjunction Ø Failure of sister chromatids (what stage?) or homologous chromosomes (what stage?) to separate during nuclear division Ø Causes genetic disorders among resulting offspring

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22 Polyploidy Ø Individuals have three or more of each type of chromosome Ø Lethal in humans Ø Many flowering plants, and some insects, fishes, and other animals, are polyploid

23 Aneuploidy Ø An individual s cells have too many or too few copies of a particular chromosome (result of non-disjunction) Ø Most cases of autosomal aneuploidy are lethal in embryos Ø Trisomy 21 (Down syndrome)

24 Change in number of sex chromosomes Ø Usually results in some degree of impairment in learning and motor skills Ø In individuals with trisomy (XXY, XXX, and XYY), these problems can be subtle and the cause may never be diagnosed

25 Sex chromosome abnormalities in females Individuals with Turner syndrome have an X chromosome and no corresponding X or Y chromosome (XO) (1/2500) Ø Well proportioned but short Ø Ovaries do not develop properly Ø Insufficient sex hormones to become sexually mature

26 Sex chromosome abnormalities in males Klinefelter syndrome (XXY ) (1/500) Ø Tend to be overweight Ø Tall Ø Normal range of intelligence Ø Make more estrogen and less testosterone than normal males, which has feminizing effects

27 How Do We Use What We Know About Human Inheritance? Genetic screening Ø Can estimate probability that a child will inherit a genetic disorder Ø Pedigrees and genotypes are analyzed by a genetic counselor Ø Some disorders can be detected early enough to start counter measures before symptoms develop Ø More than 30 conditions detectable prenatally

28 Testing of an embryo or fetus can reveal genetic abnormalities or disorders before birth Ø Obstetric sonography (safe) Prenatal diagnosis Ø Fetoscopy (2-10% chance of miscarriage) Ø Amniocentesis (14-16 weeks, presence of certain chemicals and karyotyping) Ø Chorionic villus sampling (CVS) (8-10 weeks, immediate karyotyping) Carrier Recognition Ø Sickle cell, Tay-Sachs and cystic fibrosis

29 Newborn screening for phenylketonuria (PKU) Ø Newborns screened for mutations in the gene phenylalanine hydroxylase, a defect that can cause phenylalanine to accumulate to high levels Ø Results in imbalance that inhibits protein synthesis in the brain Ø Causes severe neurological symptoms characteristic of (PKU)