Genetics: Study Of The Mechanisms Of Heredity • Mendelian Genetics proposed in mid-1800s by Gregor Mendel • determined that inherited characteristics consisted of alleles that are dominant or not • Subsequent discoveries like the material of heredity (DNA) have added to Mendel’s work but not completely replaced it! • DNA structure discovered in the 1950s by Watson and Crick • Researchers learned DNA and the sequence of ‘bases’ was the code • Location of genes, alleles • The source of inherited traits • Human Genome Project has sequenced human DNA – offering an understanding of the location of particular genes on chromosomes and the traits the genes provide code for • Genome: organism's complete set of DNA, including all of its genes. • Genes, segments of DNA, contain the codons: “recipe” or blueprints, for synthesis of proteins
Vocabulary of Genetics • Diploid number (46) of chromosomes in all cells except gametes • 1 pair of sex chromosomes that determine genetic sex (XX = female, XY = male) • 22 pairs of autosomes that guide expression of most other traits • Autosomes look alike and carry genes for the same traits but not necessarily the same expression of the trait • Karyotype: diploid chromosomal complement displayed in homologous pairs • Genetic screening and genetic counseling • Analysis of the pedigree • Fetal testing: amniocentesis, chorionic villus sampling
Notice not all chromosomes are identical in size or shape Genetically female
Genetically male
Gene Pairs: Alleles Alleles are genes that occur at same locus (location) on homologous chromosomes • DNA sequence can be the same or different • Sequence = groupings of codons that code for amino acid sequence in a single protein • “one gene, one protein” – consider the numerous functions of proteins in A&P • Homozygous: alleles are same for single trait • Heterozygous: alleles are different for single trait • i.e., Allele inherited from mom is NOT identical to allele inherited from dad but still codes for same trait
Gene Expression Dominance: one allele masks (suppresses) expression of its recessive partner • Dominant allele is denoted by capital letter and recessive by same letter, but in lowercase • Example: brown eyes is dominant trait, designated as B ; blue eyes is recessive trait designated as b • Dominant trait is expressed even if other allele codes for recessive trait • Example: BB or Bb will result in brown eyes, not blue • Recessive trait is expressed only if both alleles are recessive • Example: blue eyes occur only if person has bb • **There is no inherent benefit or superiority in dominant alleles, they are just the ones expressed**
Gene Expression • Genotype: genetic makeup of a person for a trait • For eye color example, person can have three possible genotypes: BB , Bb , bb • **Only if the trait is controlled by a single pair of alleles** • Phenotype: physical expression of genotype • Person with genotypes BB or Bb will have brown eyes ( B is dominant) • Person with genotype bb will have blue eyes • Heterozygous person with Bb has genotype for blue eyes, but phenotypically will have brown eyes • Few phenotypes can be traced to single gene Multiple allele inheritance - Most traits determined by multiple alleles or by interaction of several gene pairs (polygenetic inheritance)
Figure 29.4 Genotype and phenotype probabilities resulting from a mating of two heterozygous parents. Tt female Tt male Heterozygous Heterozygous female forms male forms two types two types of gametes of gametes T T 1/2 1/2 TT t t 1/2 1/2 1/4 tT Tt 1/4 1/4 1:2:1 genotypic ratio tt 3:1 phenotypic ratio Using a Punnett Square 25% chance homozygous 1/4 to predict frequency of Possible 50% chance heterozygous genotypes and combinations in offspring phenotypes in offspring
Gene Expression Simple dominance/recessive inheritance not as common as once thought but still…is present in the human genome Dominant traits dictated by dominant alleles • Simple examples: widow’s peaks, freckles, and dimples • Dominant disorders are uncommon because most are lethal, and death occurs before reproductive age • Exception is Huntington’s disease: not activated until ~ age 40 • Offspring of individual with Huntington’s have a 50% chance of also having disease Recessive genes may result in the more desirable condition • Example: normal vision is a recessive trait, whereas astigmatism is a dominant trait • Many genetic disorders are inherited as simple recessive traits • Examples: albinism, cystic fibrosis, and Tay-Sachs disease • Recessive nature makes them less common in occurrence • Heterozygotes are carriers of trait, meaning they do not express trait but can pass it on to offspring
Table 29.1 Traits Determined by Simple Dominant-Recessive Inheritance
Gene Expression Incomplete dominance • Heterozygous individuals have intermediate phenotype • Example: sickling gene causing abnormal hemoglobin • SS = normal hemoglobin (Hb) made • Ss = sickle-cell trait: both mutated and normal Hb are made; person can suffer sickle-cell crisis under prolonged reduction in blood O 2 • ss = sickle-cell anemia: makes only mutated Hb; person is more susceptible to sickle-cell crisis even with short O 2 reduction • FYI - One hypothesis for the presence of the s allele is that it offers the carrier some protection from the malaria blood parasite!
Gene Expression Multiple-allele inheritance • Genes that exhibit more than two allele forms • Example: ABO blood groups have three alleles: I A , I B , and I • Rh status controlled by another gene
Gene Expression Polygene inheritance • Traits that are result of actions of several gene pairs at different locations • The more genes are involved in a trait, the more phenotypic variation will be seen • Results in continuous ( quantitative ) phenotypic variation between two extremes • Examples: skin color, eye color, height, intelligence, metabolic rate Example of polygenic inheritance for skin color • Alleles for dark skin ( ABC ) are incompletely dominant over those for light skin ( abc ) • First-generation offspring of AABBCC (very dark) aabbcc (very light) cross would result in all heterozygotes with intermediate pigmentation • Second-generation offspring would have even wider variation in possible pigmentations, which, if charted, would lead to a bell-shaped curve
Figure 29.6 Simplified model for polygene inheritance of skin color based on three gene pairs. P generation: homozygous Parents dominant crossed with homozygous recessive aabbcc AABBCC (very light) (very dark) First-generation F 1 generation: two offspring AaBbCc AaBbCc heterozygous individuals 20/64 Proportion of second-generation 15/64 population F 2 generation: probability of particular phenotypes 6/64 1/64 6/64 15/64 15/64 6/64 1/64 20/64 1/64
Sex-linked Inheritance Inherited traits determined by genes on sex chromosomes • X chromosomes bear over 1400 genes • Genes found only on X chromosome are called X-linked genes • Y chromosomes carry about 200 genes Note: these genes include those that have nothing to do with the sex of the individual • X-linked recessive alleles are always expressed in males and are never masked or damped because there is no Y counterpart • Females must have recessive alleles on both X chromosomes in order to express an X-linked condition • X-linked recessive conditions are passed from mothers to sons • Example: hemophilia or red-green color blindness, types of balding • Can also be passed from mothers to daughters, but females require two alleles to express
Environmental Factors’ Effect on Gene Expression In many situations, environment can override or influence gene expression • Maternal factors can alter normal gene expression during embryonic development • Example: teratogenicity of thalidomide • Embryos developed phenotypes not directed by their genes, but by the drug • Environmental factors can also influence gene expression after birth • Poor nutrition can affect brain growth, body development, and height • Childhood hormonal deficits can lead to abnormal skeletal growth and proportions • Lead (Pb)doses in children and impacts to intellectual development
Supporting Genetic Variability 1. Mutations i. “A mutation is a change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke. Mutations can occur during DNA replication if errors are made and not corrected in time .” ii. https://courses.lumenlearning.com/wmopen-biology1/chapter/dna- mutations/ 2. Crossover of homologues during gametogenesis 3. Independent assortment of chromosomes during gametogenesis 4. Random fertilization of eggs by sperm
Figure 29.3-1 Crossover and genetic recombination. Eye color Hair color genes genes h e h e H E H E Homologous chromosomes synapse during prophase of meiosis I. Each chromosome consists of two sister chromatids. Allele for brown eyes Allele for brown hair Allele for blond hair Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome
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