Chapter 12: Inheritance Patterns and Human Genetics

Sex Determination Thomas Hunt Morgan- geneticist Studied fruit flies Drosophilia melanogaster Fruit flies have four pairs of homologous chromosomes (2n= ) Noticed one homologous pair was different in males than in females

Sex Determination Named the chromosome that appeared the same in males and females X chromosome Other chromosome was shorter and Morgan called this the Y chromosome Females are XX, males have one of each chromosome and are XY

Sex Determination Punnett Square

Sex Linkage Genes carried on X chromosomes are X- linked genes Genes carried on Y chromosome are said to be Y-linked Example: Eye color in Drosophilia

Sex Linkage Punnett Square

Linkage Groups Many genes found on a single chromosome Genes found on same chromosome make up a linkage group Crossing over can occur and create new gene combinations, but genes must be far enough apart on the chromosome

Chromosome Mapping Chromosome maps are diagrams of the gene sequence on a chromosome Genes separated by crossing over 1% of time are 1 map unit apart

Mutation 1. Germ cell mutations- occur in gametes -don’t affect organism, but can be passed on to offspring if affected gamete is fertilized 2. Somatic mutations- occur in normal cells and affect the organism—these mutations are NOT passed on to offspring Ex: skin cancer

Chromosome Mutations Deletion- loss of a piece of chromosome due to chromosomal breakage Inversion- part of chromosome breaks off and reattaches in reverse orientation Translocation- part of a chromosome breaks off and then reattaches to another nonhomologous chromosome

Chromosome mutations Nondisjunction- failure of chromosome to separate from its homologue Ex: Trisomy 21 in humans = Down Syndrome Chromosomal mutations are especially common in plants

Gene Mutations Substitution- one nucleotide of a codon is substituted with another– may or may not code for a new amino acid Frame shift mutation- results from a nucleotide being added or deleted from a gene– can have serious effects on genes

Human Genetics Pedigrees are a graphical way to show inheritance pattern in a family tree MaleFemale Affected maleAffected female Carrier Death Mating

Pedigree Example (Autosomal Recessive Inheritance)

Human Inheritance Patterns 1.Dominant Autosomal Inheritance -controlled by a single dominant allele -Ex: Huntington’s Disease Polydactyly(more than 5 fingers) -HD is a lethal disorder. So why is it passed on from generation to generation?

Human Inheritance Patterns 2.Recessive Autosomal Inheritance -controlled by a recessive allele -Ex: Cystic fibrosis, Tay Sachs Sickle cell anemia, PKU Albinism -CF affects breathing and food digestion and 1 out of 30 Americans is a carrier for CF

Human Inheritance Patterns Sickle CellNormal RBC -Sickle cells carry less O 2 and have a shorter life span than normal RBCs -Symptoms of sickle cell anemia include fatigue, shortness of breath, and tissue damage

Human Inheritance Patterns 3.Multiple Alleles -Traits controlled by three or more alleles of the same gene -Ex: Human Blood Types 4 Blood Types (A, B, AB, O) coded for by 3 different alleles (I A, I B, i) - I A and I B are codominant and both are dominant to i

Human Blood Type Punnett Square Example

Human Inheritance Patterns 4.Polygenic Inheritance -Traits are controlled by 2 or more genes Ex: skin color, eye color, height -skin and eye color are determined by amount of pigment (melanin) present -Height and other polygenic traits are controlled by environmental factors as well as genes (i.e. nutrition and disease)

Human Inheritance Patterns 5.X-linked Traits -genes for these traits are found on X chromosome (more common in males) Ex: red/green colorblindness (8% of males) hemophilia-blood clotting disorder *Not all X-linked traits are disorders or diseases, most genes on X chromosome code for necessary proteins

Human Inheritance Patterns 6.Sex-influenced Traits -traits are influenced by the presence of male or female sex hormones -Males and females have different phenotypes even though genotypes are same Ex: Pattern baldness (B) dominant in males and recessive in females

-Men and women that are BB will eventually lose their hair -B’ codes for normal, nonbald phenotype -BB’ woman will not lose her hair, but BB’ man will because of higher level of testosterone *Most sex-influenced traits are found on autosomes

Disorders Resulting from Nondisjunction Aneuploidy- having an abnormal # of chromosomes -Nondisjunction causes gametes to have lack a chromosome or have an extra one 2 Types 1.Monosomy-zygote is lacking a chromosome from one pair 2. Trisomy-zygote has extra chromosome for one pair

Disorders Resulting from Nondisjunction Abnormalities in chromosome # are often lethal Trisomy example: Down syndrome (trisomy 21) extra chromosome 21—mental retardation, facial features, heart defects Klinefelter’s syndrome (XXY)—male w/ some feminine characteristics, infertile Turner’s syndrome (XO)– female appearance, infertile (YO)– lethal condition, need genes on X

Detecting Genetic Disorders Genetic screening and counseling has become much more common Amniocentesis (14 th and 16 th week) and chronic villi sampling (8 th and 10 th week) are ways doctors can analyze DNA of fetus and create a karyotype