Chromosomes and Human Inheritance - Patterns of Inheritance

Independent Assortment

• Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes

Genes on Same Chromosome

• Linked genes

Genes on Same Chromosome

• Example of genetic linkage

– Flower color and pollen shape are on same chromosome in sweet peas

– Gene assignments• Let P = purple flowers and p = red flowers• Let L = long pollen shape and l = round shape

Recombination

• Genes on same chromosome do not always sort together

• Crossing over in Prophase I of meiosis creates new gene combinations

– Crossing over involves the exchange of DNA between chromatids of paired homologous chromosomes

Sex Chromosomes and Autosomes

• Autosomes

• Sex Chromosomes

• XX (x) XY

Sex-Linked Genes

• Genes carried on one sex chromosome are sex-linked

• X and Y chromosomes have very few genes in common

How Sex-Linkage Affects Inheritance

• Eye color genes were found to be carried by the X chromosome

– R = red eyes (dominant)– r = white eyes (recessive)

http://www.taxodros.uzh.ch/

How Sex-Linkage Affects Inheritance

• Sex-linked (specifically X-linked) recessive alleles displayed their phenotype more often in males

– Males showed recessive white-eyed phenotype more often than females in an

XRXr x XRY cross

Incomplete Dominance

• Incomplete Dominance

– When the heterozygous phenotype is intermediate between the two homozygous phenotypes

Multiple Alleles

• A species may have more than 2 alleles for a given characteristic

– Each individual still carries 2 alleles for this characteristic

– Ex. Fruit fly eye color– Ex. A, B, O blood type

Codominance

• Heterozygotes display phenotypes of both the homozygote phenotypes in codominance

• Example: Human blood group alleles

Polygenic Inheritance

– Are governed by the interaction of more than 2 genes at multiple loci

– Examples include human height, skin color, and body build, and grain color in wheat

Epigenetics

• Environment can influence how genes are expressed

• Example: Himalayan rabbit

Recessive Genetic Disorders

• New alleles produced by mutation usually code for non-functional proteins

• Alleles coding for non-functional proteins are recessive to those coding for functional ones

Recessive Genetic Disorders

• Heterozygous individuals are carriers of a recessive genetic trait

– (but otherwise have a normal phenotype)

• Homozygous recessive genes express the defective phenotype

Albinism

• Melanin is the dark pigment that colors skin cells

• Melanin is produced by the enzyme tyrosinase (TYR)

• Mutant TYR allele encodes a defective tyrosinase protein in skin cells, producing no melanin

Albinism

• Humans and other mammals who are homozygous for the mutant TYR have no skin, fur, or eye coloring (skin and hair appear white, eyes are pink)

Sickle-Cell Anemia• Hemoglobin - oxygen-transporting protein

found in red blood cells

• Mutant hemoglobin gene causes hemoglobin molecules in blood cells to clump together

– Red blood cells take on a sickle (crescent) shape and easily break

Dominant Genetic Disorders

• Dominant disease alleles disrupt normal cell function in a variety of ways

– Produce an abnormal protein that interferes with the function of the normal one

– Encode toxic proteins

– Encode a protein that is overactive or active at inappropriate times and places

Sex-Linked Genetic Disorders

• Several defective alleles for characteristics encoded on X chromosome

• Sex-linked disorders appear more frequently in males and often skip generations

• Ex. Red-green color blindness

Sex-Linked Genetic Disorders

– Hemophilia (deficiency in blood clotting protein)• Hemophilia gene in

Queen Victoria of England was passed among the royal families of Europe

• Errors in Chromosome Number

Non-Disjunction

• Incorrect separation of chromosomes or chromatids in meiosis known as non-disjunction

Abnormal Sex Chromosome Number

• Non-disjunction of sex chromosomes in males or females produce abnormal numbers of X and Y chromosomes

Abnormal Sex Chromosome Number

– Turner Syndrome (XO): an underdeveloped, infertile woman with only one X chromosome

lucinafoundation.org

Abnormal Sex Chromosome Number

– Trisomy X (XXX): a fertile, “normal” woman with an extra X chromosome

Abnormal Sex Chromosome Number

– Kleinfelter Syndrome (XXY): an infertile man with an extra X chromosome, having partial breast development and small testes

beliefnet.com

Abnormal Sex Chromosome Number

– Jacob Syndrome,

XYY Male: a tall man with an extra Y that produces high levels of testosterone and may score lower on IQ tests

biology.iupui.edu

Abnormal Autosome Number

• Non-disjunction of autosomes can occur during meiosis in the father or mother

• Frequency increases with age of parents

Abnormal Autosome Number

• Trisomy 21 (Down Syndrome)

– Individuals have 3 copies of chromosome 21

– Characterized by distinctively shaped eyelids, among other physical features

The End