Chapter 12 Patterns of Heredity & Human Inheritance

12.1: Mendelian Inheritance of Human Traits Pedigree- graphic representation of genetic inheritance Different symbols are used to represent: Males and females Individuals affected or unaffected by a particular trait Carrier- a heterozygote for a recessive trait that does have exhibit the trait in their phenotype, but “carries” the allele for the trait in his or her genotype

Recessive Disorders Disorders that appear or are expressed when the genotype is homozygous recessive

Examples of recessive disorders: 1. Cystic fibrosis Results in accumulation of thick mucus in lungs and digestive tract 2. Tay-Sachs disease Lipids are not broken down in brain cells As lipids accumulate, brain function declines and eventually leads to death Especially common on those of eastern- European Jewish descent

3. Phenylkentonuria (PKU) Amino acid phenylalanine cannot be broken down Accumulation leads to mental retardation Treated by a diet low in phenylalanine

Dominant Disorders Disorders that appear or are expressed when the genotype is heterozygous or homozygous dominant

Examples (not necessarily disorders) 1. Cleft chin, widow’s peak, hitchhiker’s thumb, mid-digit hair 2. Huntington’s disease Results in breakdown of certain areas of the brain No treatment

12.2: When heredity follows different rules Most inheritance does not follow the simple patterns established by Mendel

Incomplete Dominance Heterozygotes have an appearance that is in between the phenotypes of the two homozygotes Capital letters will be used for both traits because neither is dominant

Flower color

Example problem for incomplete dominance:

Multiple Alleles Every trait we have discussed so far has two alleles Some traits have more than two possible alleles However, each individual only has two of them (one on each of two homologous chromosomes)

A, B, O blood groups Letters refer to carbohydrates found on the surface of red blood cells Possible alleles for blood type: I A - carbohydrate A is present I B - carbohydrate B is present i- no carbohydrate is present Both I A and I B are dominant over i

Possible Genotypes and Phenotypes I A I A - type A I B I B - type B I A I B - type AB I A i- type A I B i- type B ii- type O

Example problem for multiple alleles:

Codominance Notice that when I A and I B are paired together, both alleles are expressed, blood type is AB Codominance- both alleles are expressed in heterozygotes, both alleles are dominant

Sickle Cell Disease- an example of codominance Symptoms: Breakdown of red blood cells Clumping of cells & clogging of vessels Accumulation of sickle cells in spleen Heart failure, pain, fever, brain damage, weakness, kidney failure

Genotypes & Phenotypes N- normalS- sickle cells SS- suffer from full-blown disease SN- usually healthy, but may have some symptoms at high altitudes NN- no symptoms

About 1 in 10 African Americans is a carrier for sickle cell disease, but the disease is very rare in Americans of other ancestry Why is this gene so common if the effects are so serious?

Heterozygote advantage- heterozygotes for a particular trait have some advantage that homozygotes do not In the case of sickle cell heterozygotes, they are resistant to malaria In locations where malaria is prevalent, heterozygotes tend to live longer and produce more offspring

Malaria

Example problem for sickle cell disease:

Sex Determination There are two possible sex chromosomes in humans, X or Y Females- XX Males- XY Sperm determines the sex of the offspring

Example problem for sex determination:

Sex-Linked Traits Genes located on the sex chromosomes Most sex-linked genes are located on the X chromosome- phenotype can be seen in both males and females A few are located on the Y chromosome- phenotype can only be seen in males

Sex-linked disorders Color-deficiency (blindness) Malfunction of light-sensitive cells in eyes Red-green- see gray tones instead of red or green Involves several X-linked genes Normal color vision- see over 150 colors Color-deficient- see fewer than 25 colors

Can females be colorblind? Possible alleles: X B - normalX b - color deficient Possible genotypes & phenotypes: X B X B - normal female X B Y- normal male X B X b - carrier female X b X b - color deficient female X b Y- color deficient male

Example problem for sex-linked traits:

Can males be carriers of X-linked traits? Why not?

Hemophilia X-linked gene Excessive bleeding when injured- lack clotting factor May bleed to death after minor injury Excess blood pools in joints

Duchenne muscular dystrophy Progressive weakening & loss of muscle tissue Death usually occurs by age 20

Polygenic Inheritance Polygenic inheritance- the additive effects of two or more genes appear in a single phenotypic characteristic These characteristics tend to occur on a continuum What are some examples of human characteristics that occur on a continuum?

Skin tone

External Influences on Phenotype External environment can affect the expression of certain genotypes Examples of external influences: Temperature Light Nutrition Chemicals

Internal Influences on Phenotype Gene expression may be affected by internal environments: Presence or absence of hormones Structural differences Age

12.3: Complex Inheritance of Human Traits See examples from section 12.2 notes

Karyotype Orderly display of magnified images of an individual's chromosomes Can detect chromosomal abnormalities

Abnormalities in Chromosomal Number Usually, a human embryo with an abnormal number of chromosomes is miscarried. Some abnormalities in chromosomal number upset genetic balance less drastically

Causes of chromosomal number abnormalities Nondisjunction- when members of a chromosome pair fail to separate During Meiosis I: Homologous chromosomes do not separate All gametes have abnormal numbers During Meiosis II: Sister chromatids do not separate 2 gametes are normal, 2 are abnormal

If either of these events occurs, surviving offspring that results from these abnormal gametes will have abnormal karyotypes

Disorders Resulting from Nondisjunction Trisomy 21 Result of nondisjunction in chromosome 21 Affected individuals have 3 copies of chromosome 21 Leads to Down Syndrome

Down Syndrome

Klinefelter’s Syndrome Extra X in males- XXY Sterility Breast enlargement Normal intelligence Also includes multiple disjunctions- XXYY, XXXY, XXXXY- more mental retardation

Klinefelter’s Syndrome

Jacob’s Syndrome Extra Y in males- XYY Normal males Taller than average

Superwoman Syndrome (metafemales) Extra X in females- XXX Normal females Limited fertility

Turner Syndrome Females lacking an X- XO Short stature Webbing of neck Sterile, poor development of secondary sex characteristics Normal intelligence

Turner Syndrome