Chapter 24 Genetics and Genomics

Transcription

1 Chapter 24 Genetics and Genomics Genetics study of inheritance of characteristics Genome complete set of genetic instructions Genomics field in which the body is studied in terms of multiple, interacting genes 1 Chromosomes and Genes Come in Pairs 23 pairs of chromosomes pairs 1-22 are autosomes pair 23 are sex chromosomes Normal karyotype 2 Genotype and Phenotype Genotype particular combination of genes alleles are variant forms of the same gene homozygous identical alleles heterozygous different alleles wild type allele produces most common or normal phenotype Phenotype way that genes are expressed blue eyes, presence of a protein, etc 3 1

2 Dominant and Recessive Inheritance Dominant allele masks the phenotype of the recessive allele Recessive allele is expressed only if in a double dose (homozygous) Autosomal conditions are carried on a nonsex chromosome Sex-linked conditions are carried on a sex chromosome X-linked conditions are carried on the X chromosome Y-linked conditions are carried on the Y chromosome 4 Autosomal Recessive Disorder cystic fibrosis is an example sexes are affected with equal frequencies offspring probabilities 25% homozygous dominant 50% heterozygous 25% homozygous recessive punnet square and a pedigree are useful ways to express genetic information 5 Autosomal Dominant Disorder Huntington disease is an example a person with one HD allele develops the disease both sexes are equally affected 6 2

3 Different Dominance Relationships: Incomplete Dominance heterozygote has a phenotype intermediate between homozygous dominant and homozygous recessive familial hypercholesterolemia is an example 7 Different Dominance Relationships: Codominance different alleles are both expressed ABO blood type is an example three alleles of ABO blood typing are I A, I B, I a person with type A may have the genotype I A i or I A I A a person with type B may have the genotype I B i or I B I B a person with type AB must have the genotype I A I B a person with type O blood must have the genotype ii 8 Gene Expression: Penetrance and Expressivity Complete penetrance everyone who inherits the disease causing alleles has some symptoms Imcomplete penetrance some individuals do not express the phenotype even though they inherit the alleles (example polydactyly) Variable expression symptoms vary in intensity in different people two extra digits versus three extra digits in polydactyly 9 3

4 Gene Expression: Pleiotrophy and Genetic Heterogeneity Pleiotropy single genetic disorder producing several symptoms Marfan syndrome is an example people affected produce several symptoms that vary Genetic Heterogeneity same phenotype resulting from the actions of different genes hereditary deafness is an example 10 Complex Traits Most if not all characteristics and disorders considered inherited actually reflect input from the environment as well as genes. Polygenic traits determined by more than one gene height, skin color, eye color Multifactorial traits traits molded by one or more genes plus environmental factors height and skin color 11 Variations in Height 12 4

5 Variations in Skin Color Mid-range colors are more common 13 Sex Determination an egg contributes an X chromosome a sperm contributes either an X or a Y chromosome A gene on the Y chromosome, SRY, determines sex 14 Sex Chromosomes and Their Genes X chromosome has over 1,500 genes most genes on the X chromosome do not have corresponding alleles on the Y chromosome Y chromosome has only 231 protein-encoding genes some genes are unique only to the Y chromosome 15 5

6 Sex-linked Genes Y-linked genes are transmitted only from father to son X-linked genes are transmitted from father to daughter or from mother to daughter or son Hemophilia A is a sex-linked disorder 16 Hemophilia A passed from mother (heterozygote) to son each son has a 50% chance of receiving the recessive allele from the mother each son with one recessive allele will have the disease each son has no allele on the Y chromosome to mask the recessive allele each daughter has a 50% chance of receiving the recessive allele from the mother each daughter with one recessive allele will be a carrier each daughter with one recessive allele does not develop the disease because she has another X chromosome with a dominant allele 17 Gender Effects on Phenotype Sex-limited trait affects a structure or function of the body that is present in only males or only females examples are beards or growth of breasts Sex-influenced inheritance an allele is dominant in one sex and recessive in the other baldness is an example heterozygous males are bald but heterozygous females are not 18 6

7 Chromosomal Disorders Polyploidy extra set of chromosomes most embryos die Aneuploidy missing a chromosome or having an extra chromosome results from nondisjunction trisomy is the condition of having an extra chromosome monosomy is the condition of missing a chromosome Euploid is a normal chromosome number 19 Causes of Aneuploidy 20 Prenatal Tests 21 7

8 Prenatal Tests 22 Gene Therapy group of experimental techniques that alter, replace, silence, or augment a gene s function to improve, delay or prevent symptoms heritable gene therapy introduces the genetic change into a sperm, egg, or zygote changes passed to future generations common in plants; not done in humans nonheritable gene therapy targets only affected cells changes not passed to future generations 23 Gene Therapy Targets 24 8

9 Clinical Application Down Syndrome most common autosomal aneuploid trisomy 21 signs and symptoms include short stature mental retardation protruding tongue heart defects kidney defects suppressed immune systems digestive disorders 25 9