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)

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 X R X r x X R Y cross

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