HEREDITY

THE VOCABULARY OF GENETICS Introduction: The nuclei of all human cells except gametes contain the diploid number of chromosomes (46) or 23 pairs of homologous chromosomes Two of the 46 chromosomes, and the remaining 44 are 23 pairs of autosomes that guide the expression of most other traits Homologous chromosomes are the two chromosomes that make up a pair in a diploid cell, one from the egg and one from the sperm The karotype is the diploid chromosomal complement displayed in homologous pairs

PREPARING A HUMAN KARYOTYPE

THE VOCABULARY OF GENETICS Chromosomes are paired, with one coming from each parent, and the genes on those chromosomes are also paired Alleles are any two matched genes at the same locus (site) on homologous chromosomes Homozygous means that the two alleles are the same Heterozygous means the two alleles are different A dominant allele is one that will mask or suppress the expression of the other allele A recessive allele is the allele that is suppressed by the dominant allele and will only be expressed in the homozygous condition Genotype and Phenotype A person’s genetic makeup is called his or her genotype The way the genotype is expressed in the body is that individual’s phenotype

SEXUAL SOURCES OF GENETIC VARIATION During metaphase of meiosis I the alignment of the tetrads along the center of the cell is completely random, allowing for the random distribution of material and paternal chromosomes into the daughter cells In meiosis I the two alleles determining each trait are segregated, or distributed to different gametes Alleles on different pairs of homologous chromosomes are distributed independently of each other Independent assortment of homologues during meiosis I can be calculated as 2n, where n is the number of homologous pairs During meiosis I homologous chromosomes may exchange gene segments, a process called crossing over, which gives rise to recombinant chromosomes that have contributions from each parent Random fertilization occurs because a single human egg is fertilized by a single human sperm in a completely haphazard basis

INDEPENDENT ASSORTMENT OF HOMOLOGOUS CHROMOSOMES DURING METAPHASE OF MEIOSIS I

CROSSING OVER AND GENETIC RECOMBINATION OCCURRING DURING MEIOSIS I

TYPES OF INHERITANCE Dominant-Recessive Inheritance A Punnett square is used to determine the possible gene combinations resulting from the mating of parents of known genotypes Dominant traits include widow’s peaks, dimples, freckles, and detached earlobes Genetic disorders caused by dominant genes are uncommon because lethal dominant genes are always expressed and result in the death of the embryo, fetus, or child Recessive traits include some desirable conditions such as normal vision, but they also include most genetic disorders Some traits exhibit incomplete dominance, where the heterozygote has a phenotype intermediate between those of the homozygous dominant and the homozygous recessive Some genes exhibit more than two allele forms, such as blood type, leading to a phenomenon called multiple-allele inheritance

PROBABILITIES OF GENOTYPES AND PHENOTYPES RESULTING FROM A MATING OF TWO HETEROZYGOUS PARENTS

TYPES OF INHERITANCE Inherited traits determined by genes on the sex chromosomes are said to be sex-linked The Y chromosome is much smaller than the X chromosome and lacks many of the genes present on the X that code for nonsexual characteristics, such as red-green color blindness Genes found only on the X chromosome are said to be X-linked; those found only on the Y chromosome, such as those that code for maleness, are said to be Y-linked Polygene inheritance results in continuous or quantitative phenotype variation between two extremes, and depends on several gene pairs at different locations acting in tandem (an example of this is skin color)

POLYGENE INHERITANCE OF SKIN COLOR BASED ON THREE SEPARATE GENE PAIRS

ENVIRONMENTAL FACTORS IN GENE EXPRESSION In many situations environmental factors override or at least influence gene expression Phenocopies are environmentally produced phenotypes that mimic conditions that may be caused by genetic mutations Environmental factors may also influence genetic expression after birth, such as the effect of poor infant nutrition on brain growth, body development, and height

NONTRADITIONAL INHERITANCE Genomic imprinting somehow tags genes during gametogenesis as either paternal or maternal and confers important functional differences in the resulting embryo Disorders have been traced to errors in mitochondrial genes that are transmitted to the offspring almost exclusively by the mother

GENETIC SCREENING, COUNSELING, AND THERAPY When one of the parents of a developing embryo displays a recessive disorder it is important to determine if the other partner is a heterozygote and thus a carrier for that trait One way to identify if an individual may be a carrier for a particular trait is to do a pedigree, which traces a genetic trait through several generations Blood tests, both simple and a more sophisticated blood chemistry test, and DNA probes can be used to detect the presence of unexpressed recessive genes

PEDIGREE

GENETIC SCREENING, COUNSELING, AND THERAPY Fetal testing is used when there is a known risk of a genetic disorder The most common type of fetal testing is amniocentesis, where a needle is inserted into the amniotic sac to withdraw amniotic fluid for testing Chorionic villi sampling (CVS) suctions off bits of the chorionic villi from the placenta for examination Human gene therapy and genetic engineering have the potential to alleviate or even cure diseases, especially those traced to one defective gene

FETAL TESTING