Chapter 30 Heredity

Genetics The study of the mechanism of heredity Nuclei of all human cells (except gametes) contain 46 chromosomes Sex chromosomes determine the genetic sex (XX = female, XY = male) Karyotype – the diploid chromosomal complement displayed in homologous pairs Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal

Alleles Matched genes at the same locus on homologous chromosomes Homozygous – two alleles controlling a single trait are the same Heterozygous – the two alleles for a trait are different Dominant – an allele masks or suppresses the expression of its partner Recessive – the allele that is masked or suppressed

Genotype and Phenotype Genotype – the genetic makeup Phenotype – the way one’s genotype is expressed

Segregation and Independent Assortment Chromosomes are randomly distributed to daughter cells Members of the allele pair for each trait are segregated during meiosis Alleles on different pairs of homologous chromosomes are distributed independently The number of different types of gametes can be calculated by this formula: 2n, where n is the number of homologous pairs In a man’s testes, the number of gamete types that can be produced based on independent assortment is 223, which equals 8.5 million possibilities

Segregation and Independent Assortment Figure 30.2

Crossover Homologous chromosomes synapse in meiosis I One chromosome segment exchanges positions with its homologous maternal counterpart Crossing over occurs, forming a chiasma Genetic information is exchanged between homologous chromosomes Two recombinant chromosomes are formed

Crossover Figure 30.3.1

Crossover Figure 30.3.2

Random Fertilization A single egg is fertilized by a single sperm in a random manner Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million  8.5 million) zygote possibilities

Dominant-Recessive Inheritance Reflects the interaction of dominant and recessive alleles Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype Example: probability of different offspring from mating two heterozygous parents T = tongue roller and t = cannot roll tongue

Dominant-Recessive Inheritance Figure 30.4

Dominant and Recessive Traits Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease Examples of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease Carriers – heterozygotes who do not express a trait but can pass it own to their offspring

Incomplete Dominance Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive Sickling is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s) SS = normal Hb is made Ss = sickle-cell trait (both aberrant and normal Hb is made) ss = sickle-cell anemia (only aberrant Hb is made)

Multiple-Allele Inheritance Genes that exhibit more than two alternate alleles ABO blood grouping is an example Three alleles (IA, IB, i) determine the ABO blood type in humans IA and IB are codominant (both are expressed if present), and i is recessive

Sex-Linked Inheritance Inherited traits determined by genes on the sex chromosomes X chromosomes bear over 2500 genes; Y carries about 15 X-linked genes are: Found only on the X chromosome Typically passed from mothers to sons Never masked or damped in males since there is no Y counterpart

Polygene Inheritance Depends on several different gene pairs at different loci acting in tandem Results in continuous phenotypic variation between two extremes Examples: skin color, eye color, and height

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 Figure 30.5

Environmental Influence on Gene Expression Phenocopies – environmentally produced phenotypes that mimic mutations Environmental factors can influence genetic expression after birth Poor nutrition can effect brain growth, body development, and height Childhood hormonal deficits can lead to abnormal skeletal growth

Genomic Imprinting The same allele can have different effects depending upon the source parent Deletions in chromosome 15 result in: Prader-Willi syndrome if inherited from the father Angelman syndrome if inherited from the mother During gametogenesis, certain genes are methylated and tagged as either maternal or paternal Developing embryos “read” these tags and express one version or the other

Genomic Imprinting Figure 30.6

Extrachromosomal (Mitochondrial) Inheritance Some genes are in the mitochondria All mitochondrial genes are transmitted by the mother Unusual muscle disorders and neurological problems have been linked to these genes

Genetic Screening, Counseling, and Therapy Newborn infants are screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and PKU Genetic screening alerts new parents that treatment may be necessary for the well-being of their infant Example: a woman pregnant for the first time at age 35 may want to know if her baby has trisomy-21 (Down syndrome)

Genetic Screening, Counseling, and Therapy Figure 30.7

Carrier Recognition Identification for the heterozygote state for a given trait Two major avenues are used to identify carriers: pedigrees and blood tests Pedigrees trace a particular genetic trait through several generations; helps to predict the future Blood tests and DNA probes can detect the presence of unexpressed recessive genes Sickling, Tay-Sachs, and cystic fibrosis genes can be identified by such tests

Fetal Testing Is used when there is a known risk of a genetic disorder Amniocentesis – amniotic fluid is withdrawn after the 14th week and sloughed fetal cells are examined for genetic abnormalities Chorionic villi sampling (CVS) – chorionic villi are sampled and karyotyped for genetic abnormalities

Fetal Testing Figure 30.9

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 The patient’s cells can be directly injected with “corrected” DNA