Genetics & Human Inheritance

Transcription

1 Genetics & Human Inheritance BIO 105 Chapter 20 Vocabulary Alleles alternate forms of a gene Trait some characteristic Homozygous individuals that contain two copies of the same allele Heterozygous individuals with different alleles of the same gene Vocabulary Dominant allele an allele whose effects can be detected, regardless of the presence of other alleles (upper case) Recessive allele an allele whose effects are masked in the heterozygous condition (lower case) Genotype set of alleles an individual possesses for a particular trait Phenotype observable physical trait 1

2 Law of Segregation Because the alleles for each gene segregate during Meiosis, half the gametes bear one allele, and half bear the other. Law of Independent Assortment Alleles for genes on different chromosomes segregate into gametes independently. 2

3 Gregor Mendel How to predict the outcome of onetrait crosses 1. Identify the possible gametes that each parent can produce 2. Use a Punnett square to determine the probable outcome of the genetic cross This diagram predicts the outcome of a particular cross or breeding experiment With this you can generate a summary of every possible combination of maternal and paternal alleles for a gene Freckles Monohybrid cross a cross in which both parents are heterozygous for one trait of interest 3

4 How to predict the outcome of twotrait crosses 1. Identify the possible gametes that each parent can produce 2. Use a Punnett square to determine the probable outcome of the genetic cross Types of inheritance 1. Dominant vs. recessive 2. Codominance 3. Incomplete dominance 4. Polygenic inheritance 5. X-linked inheritance 4

5 What makes an allele dominant or recessive? A dominant allele codes for a normal, functional protein, whereas a recessive allele codes for either a nonfunctional form of the protein or no protein at all. i.e. tyrosinase necessary for melanin formation Autosomal Recessive Disorders These are disorders that are controlled by DNA on any of the 22 autosomes These are recessive disorders so they are only expressed if the person is homozygous for the recessive allele Examples include: cystic fibrosis, albinism, phenylketonuria Cystic fibrosis 5

6 Albinism Autosomal Dominant Disorders These disorders are also controlled by the nonsex chromosomes These disorders will be expressed if the person has one dominant allele. Examples include: Huntington disorder, Marfan syndrome Huntington s Disease It is a degenerative disease that affects the cerebral cortex of the brain. Initial symptoms are abrupt, jerky movements which typically develop in middle age. Late in the disease dementia occurs. 6

7 Marfan syndrome Codominance Codominance occurs when more than one allele produces a functional protein. In this case, the effects of both alleles are separately apparent in a heterozygote. Blood Types 3 alleles: I A I B i 7

8 Blood Types Genotype Phenotype Blood Type AB (I A I B ) AA (I A I A ) BB (I B I B ) OO (ii) BO (I B i) AO (I A i) A & B antigens A antigens B antigens No A or B antigens B antigens A antigens AB A B O B A Incomplete Dominance: Sickle-cell hemoglobin 8

9 Pleiotropy Polygenic Inheritance Multiple genes affect expression of a certain trait. X-Linked Inheritance in Humans X-linked genes have no homologous allele on the Y chromosome, so their inheritance is a bit different. Males only have one X chromosome, so if they inherit a recessive allele on the X chromosome they will express the trait. If females inherit one recessive allele, they have a chance to also inherit the dominant allele. 9

10 Hemophilia Prenatal testing for certain genetic disorders Amniocentesis Chorionic villus sampling Phenylketonuria - newborns Non-functional phenylalanine hydroxylase Microcephaly Intellectual disability Seizures Behavior Skin rashes 10