Biology p. 267-286 Incomplete Dominance, Penetrance and Mitosis

• Somatic cells (nonreproductive body cells) are diploid (2n)
• Gametes such as sperm and ova (eggs) are haploid
• Polyploid organisms have more than two chromosome sets per cell
• Sexual reproduction: haploid gametes join via fertilization to produce a diploid zygote.
• Haploid gametes are produced by meiosis, which divides diploid parental cells

Meiosis

• Meiotic cell division has two major stages: meiosis I and meiosis II
• Each stage in Meiosis has four phases: prophase, metaphase, anaphase, and telophase
• During prophase I, homologous chromosomes pair side by side (synapsis).
• Crossing over takes place during prophase I
• Paired homologous chromosomes align at the metaphase plate in metaphase I
• During anaphase I the members of each homologous pair separate to opposite poles
• Unlike anaphase of mitosis, chromosomal sister chromatids do not separate during meiotic anaphase I
• Telophase I and cytokinesis result in two haploid daughter cells
• Chromosomes of haploid daughter cells are still composed of two connected sister chromatids
• Meiosis II proceeds similarly to mitosis, but occurs only in haploid cells
• During meiosis II, sister chromatids separate to form distinct chromosomes that move to opposite poles during anaphase II
• Following completion of telophase II and cytokinesis, haploid gametes are produced

Genetic Diversity

• Offspring from sexual reproduction are genetically diverse
• Haploid gametes that form offspring are themselves genetically diverse
• Gamete diversity arises during meiosis
• One feature of meiosis that generates genetic diversity is independent assortment of homologous chromosomes
• Pairs of homologous chromosomes align at the metaphase plate during metaphase I
• The orientation of each pair in relation to the cellular pole is random
• Daughter cells at the end of meiosis I have an equal chance (50%) of containing either the maternal or paternal chromosome
• Each haploid gamete produced contains a random combination of maternal and paternal chromosomes because the movement of each pair of homologous chromosomes during meiosis is independent of the movement of every other pair
• Independent assortment of homologous chromosomes into gametes contributes to gametes' genetic diversity, creating unique genetic information based on the difference in the parental chromosomes
• Meiosis involves crossing over, which results in genetic recombination by exchanging corresponding DNA segments between paired homologous chromosomes

Crossing Over

• Crossing over occurs during prophase I
• Homologous chromosomes undergo synapsis during prophase I
• Each chromosome consists of two sister chromatids
• The adjacent alignment of a homologous chromosome pair is called a tetrad because it consists of four chromatids
• A synaptonemal complex forms between homologous chromosomes and holds them tightly together forming tetrads
• The tetrad structure allows contact between the paternal and maternal chromosomes of a homologous pair
• Equivalent segments of maternal and paternal chromatids are exchanged between the chromosomes
• Exchanging DNA is a hallmark of crossing over and results in the formation of recombinant chromosomes, consisting of DNA from both parents

Comparing Meiosis and Mitosis

• Meiosis and mitosis are forms of cell division wherein cells reproduce
• Meiosis results in haploid gametes (egg, sperm)
• Mitosis occurs in somatic (non-sex) cells for organism growth and repair
• Mitosis occurs typically in diploid cells, whereas meiosis II occurs only in haploid cells
• In meiosis II, sister chromatids separate to form distinct chromosomes that move to opposite poles of the cell during anaphase II
• Following completion of telophase II and cytokinesis, haploid gametes are produced.

Key differences between the two processes:

• Meiosis: 2 rounds of cell division, resulting in 4 daughter cells
• Mitosis: 1 round of cell division, resulting in 2 daughter cells
• Ploidy of daughter cells of meiosis are haploid
• Daughter cells of mitosis are diploid (same as parent cell)
• Synapsis and crossing over between homologous chromosomes occurs in meiosis, but not mitosis
• Meiosis generates genetic diversity, but mitosis does not
• Primary purpose of meiosis is gamete production

Mendelian Concepts

• Heredity (genetic inheritance) involves the mechanisms by which genetically based traits are passed from one generation to the next

Genes and Alleles

• Genetic information is transmitted across generations from parents to offspring
• The gene is the basic unit of genetic information, which is a sequence of DNA nucleotides encoding a functional product, such as a protein or RNA molecule
• Genes are carried on chromosomes
• A gene's specific site on a chromosome is called a locus
• A chromosome may contain thousands of genetic loci
• Genes can be present in alternative forms, called alleles
• Inheritance of different alleles for a particular gene by different individuals can result in different forms of the trait controlled by the gene (eg, flower color)
• The allele that produces the most commonly observed form of a trait in a natural population is called the wild-type allele
• Mutant alleles often produces altered or abnormal forms of a trait

Genotype and Phenotype

• Organisms inherit genetic information from their parents
• An organism's genotype is its genetic information
• Diploid organisms (e.g., humans) usually possess two alleles for each gene
• Genotype refers to the two alleles present at a particular genetic locus
• Homozygous: both alleles at a locus are identical
• Heterozygous: two different alleles are present at a locus
• An organism's genetic makeup (genotype) is expressed to produce its observable physical characteristics, or phenotype
• Phenotype includes pattern of development, molecular composition, appearance, behavior, and can be influenced by environmental factors
• Alleles may be dominant or recessive, with dominant alleles represented by capital letters and recessive alleles by lowercase letters
• Dominant alleles at a locus always affect the phenotype
• Phenotypic effects of recessive alleles are apparent only when no dominant alleles are present at the locus

Phenotypes

• Dominant phenotypes expressed when homozygous dominant or heterozygous genotypes are exhibited
• Recessive phenotypes are expressed only by organisms that have homozygous recessive genotypes
• Diploid organisms possess two full sets of chromosomes and their genotype is composed of two alleles

Alleles and Phenotypes

• More than one allele exhibiting complete dominance may exist for certain genes in the gene pool
• Alleles exhibiting complete dominance fully expressed in phenotype
• When an organism inherits two different dominant alleles for a particular gene, both alleles can be fully and independently expressed
• Situation in which the phenotype shows the full contribution of two different dominant alleles in a heterozygous genotype is called codominance
• Incomplete dominance is when neither allele in heterozygotes is fully expressed
• When alleles exhibit incomplete dominance are present, heterozygous individuals exhibit a blended phenotype intermediate to the phenotypes produced by homozygous dominant and homozygous recessive individuals.

Penetrance and Expressivity

• An organism's phenotype depends on the organism's genotype, environmental conditions, and other factors like diet
• The penetrance of a genotype is the proportion of individuals with the genotype that express the expected phenotype
• A genotype that expresses the expected phenotype in 100% of the individuals is said to be fully penetrant (ie, have complete penetrance)
• When less than 100% of the individuals express the phenotype, the genotype is said to have incomplete or reduced penetrance
• Variable expressivity is the ability of a single genotype to produce a range of degrees of expression of the expected phenotype among individuals with the genotype

Genetic Crosses

• Experimental genetic crosses allow researchers to observe patterns of inheritance across generations
• A (monohybrid) genetic cross starts by "mating" organisms of known genotypes that differ phenotypically in a single trait
• The first cross usually involves homozygous parent organisms (true-breeding) for particular alleles
• The organisms true-breeding for particular alleles represent the parental (P) generation
• Offspring from the P generation cross are first filial (F1) generation
• These are all heterozygous individuals that express the dominant phenotype (resembling one P parent) only
• Mating among the F1 generation produces offspring that represent the second filial (F2) generation
• Approximately 75% of F2 generation expressing the dominant trait and approximately 25% expressing the recessive trait

Punnett Squares and Test Crosses

• A Punnett square is a graphical tool used to predict the distribution of alleles provided by two individuals with known genotypes involved in a genetic cross
• A Punnett square is set up by writing each possible combination of alleles in maternal gametes along the top of a grid
• Each combination of alleles found in paternal gametes down the left (or vice versa)
• Then, squares of the grid are filled with the corresponding maternal and paternal contributions
• Each square represents an equally probable outcome (ie, offspring genotype)
• A testcross can be used to determine whether an organism exhibiting a dominant phenotype is homozygous or heterozygous for the dominant allele
• Recessive alleles are phenotypically masked by dominant alleles in heterozygotes
• Therefore, organisms with homozygous dominant genotypes and organisms with heterozygous genotypes typically exhibit the same phenotype
• In a testcross, an organism exhibiting the dominant phenotype is mated to an organism exhibiting the recessive phenotype
• If all offspring exhibit the dominant phenotype, the parent exhibiting the dominant phenotype is homozygous
• It is likely that the parent exhibiting the dominant phenotype is heterozygous if half the offspring will have the recessive phenotype

Pedigree Analysis

• A pedigree diagrams the occurrence of a trait across multiple generations of a family
• Females are represented by circles and males by squares
• Horizontal lines between individuals show matings
• Offspring are identified through a vertical line from the horizontal line and have shading to show if they are affected

Explore incomplete dominance, where heterozygotes show intermediate phenotypes. Understand penetrance as the proportion expressing a trait, and variable expressivity, where traits vary in intensity. Review the roles of P and F1 generations and mitosis.