Copyright © 2010 Pearson Education, Inc. Genetics The study of the mechanism of heredity Basic principles proposed by Mendel in the mid-1800s

Copyright © 2010 Pearson Education, Inc. Genetics Diploid number of chromosomes In all cells except gametes Diploid number = 46 (23 pairs of homologous chromosomes) (For Humans!) 1 pair of sex chromosomes XX = female, XY = male 22 pairs of autosomes

Copyright © 2010 Pearson Education, Inc. (b) The photograph is entered into a computer, and the chromo- somes are electronically rearranged into homologous pairs according to size and structure. (c) The resulting display is the karyotype, which is examined for chromosome number and structure. (a) The slide is viewed with a microscope, and the chromosomes are photographed. Karyotype: diploid chromosomal complement displayed in homologous pairs Chromosomes & the Genome

Copyright © 2010 Pearson Education, Inc. Genotype and Phenotype Genome all the chromosomes & genes of an organism Genotype the particular set of gene versions an organism has Phenotype the physical/biochemical manifestation of the genes

Copyright © 2010 Pearson Education, Inc. Genetic Terms H locus E locus loci alleles centromeres homologous chromosomes sister chromatids

Copyright © 2010 Pearson Education, Inc. Alleles A version of a particular gene Set of genes at the same locus on homologous chromosomes Homozygous genotype Both alleles at a locus are identical Heterozygous The alleles at a locus are different

Copyright © 2010 Pearson Education, Inc. Genetic Terms Inheritance Patterns Dominant A trait appears in the phenotype even if only one allele for that trait is present Recessive A trait that can be masked Only appears in phenotype if it is homozygous

Copyright © 2010 Pearson Education, Inc. Genetic Terms Inheritance patterns Simple dominance One allele is dominant one is recessive Thumbs, blood type Co-dominance Both alleles affect a trait – each give different effect AB Blood type Intermediate dominance Two alleles give full effect, one allele gives half effect haploinsufficiency

Copyright © 2010 Pearson Education, Inc. Sexual Sources of Genetic Variation Independent assortment of chromosomes Crossover between homologues

Copyright © 2010 Pearson Education, Inc. Segregation and Independent Assortment During gametogenesis, maternal and paternal chromosomes are randomly distributed to daughter cells The two alleles of each pair are segregated during meiosis I Alleles on different pairs of homologous chromosomes are distributed independently

Copyright © 2010 Pearson Education, Inc. Segregation and Independent Assortment The number of gamete types = 2 n, where n is the number of homologous pairs In a man’s testes, 2 n = 2 23 = 8.5 x 10 6

Copyright © 2010 Pearson Education, Inc. Meiotic Recombination Genes on the same chromosome are linked Chromosomes can cross over and exchange segments Recombinant chromosomes have mixed contributions from each parent

Copyright © 2010 Pearson Education, Inc. Figure 29.3 (1 of 4) H Allele for brown hair h Allele for blond hair E Allele for brown eyes e Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair Hair color genesEye color genes Homologous chromosomes synapse in prophase of meiosis I

Copyright © 2010 Pearson Education, Inc. Figure 29.3 (2 of 4) H Allele for brown hair h Allele for blond hair E Allele for brown eyes e Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair Non-sister chromatids undergo recombination (crossing over) Chiasma

Copyright © 2010 Pearson Education, Inc. Figure 29.3 (3 of 4) H Allele for brown hair h Allele for blond hair E Allele for brown eyes e Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair The chromatids break and rejoin

Copyright © 2010 Pearson Education, Inc. Figure 29.3 (4 of 4) H Allele for brown hair h Allele for blond hair E Allele for brown eyes e Allele for blue eyes Paternal chromosome Maternal chromosome Homologous pair At end of meiosis, each haploid gamete has 1 of the 4 chromosomes shown. 2 chromosomes are recombinant – new combinations of genes Gamete 1 Gamete 2 Gamete 3 Gamete 4

Copyright © 2010 Pearson Education, Inc. Types of Inheritance Most traits are determined by multiple alleles or by the interaction of several gene pairs

Copyright © 2010 Pearson Education, Inc. Simple Dominance Inheritance Reflects the interaction of dominant and recessive alleles Punnett square Tool to represent genetic crosses Helps predict possible gene combinations resulting from the mating

Copyright © 2010 Pearson Education, Inc. Dominant-Recessive Inheritance Example: probability of genotypes from mating two heterozygous parents Dominant allele—capital letter; recessive allele—lowercase letter T = tongue roller and t = cannot roll tongue TT and tt are homozygous; Tt is heterozygous

Copyright © 2010 Pearson Education, Inc. Dominant-Recessive Inheritance Offspring: 25% TT, 50% Tt, 25% tt The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values

Copyright © 2010 Pearson Education, Inc. Figure 29.4 femalemale Heterozygous male forms two types of gametes Heterozygous female forms two types of gametes Possible combinations in offspring 1/2 1/4 1/2

Copyright © 2010 Pearson Education, Inc. Simple Dominance Inheritance Dominant traits (for example, widow’s peaks, freckles, dimples) Polydactyly, Marfan Syndrome Huntington’s disease is caused by a delayed- action gene

Copyright © 2010 Pearson Education, Inc. Simple Dominance Inheritance Simple dominance inheritance patterns frequently cause disease only when the genomtype is homozygous for the recessive alleles Albinism, cystic fibrosis, and Tay-Sachs disease, sickle-cell disease Heterozygotes have one mutant allele and one normal allele but do not have disease

Copyright © 2010 Pearson Education, Inc. What is Dominant? What is Recessive? Dominant alleles usually encode functional proteins Recessive alleles often encode defective proteins or no protein at all.

Copyright © 2010 Pearson Education, Inc. Multiple-Allele Inheritance Genes that exhibit more than two allele forms ABO blood grouping is an example Three alleles (I A, I B, i) determine the ABO blood type in humans I A and I B are codominant (both are expressed if present), and i is recessive

Copyright © 2010 Pearson Education, Inc. Table 29.2

Copyright © 2010 Pearson Education, Inc. Sex-Linked Inheritance Inherited traits determined by genes on the sex chromosomes X chromosomes > 2500 genes Y chromosomes ~80 genes

Copyright © 2010 Pearson Education, Inc. Sex-Linked Inheritance e.g. X-linked genes clotting factor (hemophilia) red opsin (red-green color blindness) Females have two X – so normal allele masks mutant Males – 1 X so mutant is expressed if present

Copyright © 2010 Pearson Education, Inc. colorblindness protanopia deuteranopia tritanopia

Copyright © 2010 Pearson Education, Inc.

Polygenic Inheritance Depends on several different genes at acting in concert Gives more fine-scale phenotypic variation between two extremes Examples: skin color, eye color, height Quantitative traits QTLs

Copyright © 2010 Pearson Education, Inc. Eye color variation

Copyright © 2010 Pearson Education, Inc. Polygenic Inheritance of Skin Color Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc) The first-generation offspring each have three “units” of darkness (intermediate pigmentation) The second-generation offspring have a wide variation in possible pigmentations

Copyright © 2010 Pearson Education, Inc. Environmental Factors in Gene Expression Phenocopies: environmentally produced phenotypes that mimic conditions caused by genetic mutations Environmental factors can influence genetic expression after birth Poor nutrition can affect brain growth, body development, and height Childhood hormonal deficits can lead to abnormal skeletal growth and proportions

Copyright © 2010 Pearson Education, Inc. Non-Mendelian Inheritance Influences due to Genes of small RNAs Epigenetic marks (chemical groups attached to DNA) Extranuclear (mitochondrial) inheritance

Copyright © 2010 Pearson Education, Inc. Small RNAs MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) Bind to mRNAs Cause mRNA to be destroyed or not translated into proteins In future, RNA-interfering drugs may treat diseases such as age-related macular degeneration and Parkinson’s disease

Copyright © 2010 Pearson Education, Inc. Extranuclear (Mitochondrial) Inheritance 37 genes are in human mitochondrial DNA (mtDNA) Transmitted by the mother in the cytoplasm of the egg Errors in mtDNA are linked to rare disorders: muscle disorders and neurological problems, possibly Alzheimer’s and Parkinson’s diseases

Copyright © 2010 Pearson Education, Inc. Genetic Screening, Counseling, and Therapy Newborn infants are routinely screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, PKU and other metabolic disorders Other examples: screening adult children of parents with Huntington’s disease: for testing a woman pregnant for the first time after age 35 to see if the baby has trisomy-21 (Down syndrome)

Copyright © 2010 Pearson Education, Inc. Carrier Recognition Two major avenues for identifying carriers of genes: pedigrees and DNA testing Pedigrees trace a particular genetic trait through several generations; helps to predict the future

Copyright © 2010 Pearson Education, Inc. Pedigree Symbols

Copyright © 2010 Pearson Education, Inc. Dominant or Recessive

Copyright © 2010 Pearson Education, Inc. Autosomal Recessive

Copyright © 2010 Pearson Education, Inc. X-linked recessive

Copyright © 2010 Pearson Education, Inc. Figure 29.7 Male Female Affected maleMating Offspring 1st generation grandparents 2nd generation (parents, aunts, uncles) 3rd generation (two sisters) Affected female Key Widow’s peak No widow’s peak Ww ww Ww ww

Copyright © 2010 Pearson Education, Inc. Carrier Recognition DNA probes can detect the presence of unexpressed recessive mutations Tay-Sachs and cystic fibrosis mutations can be identified by such tests

Copyright © 2010 Pearson Education, Inc. Fetal Testing Used when there is a known risk of a genetic disorder Amniocentesis: amniotic fluid is withdrawn after the 14th week and fluid and cells are examined for genetic abnormalities Chorionic villus sampling (CVS): chorionic villi are sampled and karyotyped for genetic abnormalities

Copyright © 2010 Pearson Education, Inc. Figure 29.8 (a) Amniocentesis Centrifugation Cervix Fluid Fetal cells Several weeks Biochemical tests Uterus Placenta Fetus Amniotic fluid withdrawn Karyotyping of chromosomes of cultured cells A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. 2 Biochemical tests can be performed immediately on the amniotic fluid or later on the cultured cells. 3 Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. 1

Copyright © 2010 Pearson Education, Inc. Human Gene Therapy Genetic engineering has the potential to replace a defective gene Defective cells can be infected with a genetically engineered virus containing a functional gene