Chromosomes and Human Inheritance - Patterns of Inheritance
description
Transcript of 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
• 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-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
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
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