Modes of Inheritance Adapted by Ellen G. Dow for QBIC Genetics Lab 2017 I. The Laws of Mendelian inheritance Learning objectives: Determine phenotypes and genotypes of organisms in different scenarios. Construct a Punnett s square using both monohybrid and dihybrid crosses. Calculate expected genotypic and phenotypic ratios. Terminology: Gene: molecular unit of inheritance that is coded by nucleic acids Allele: variations of one gene Genotype: composition of alleles for a gene or several genes that codes for a phenotype Phenotype: the physical appearance of an organism determined by the genotype and interactions with external factors, such as environment, dependent on the mode of inheritance Law of Random Segregation: Diploid germ-line cells of sexually reproducing species have two copies for (nearly) all chromosomal genes. These copies are located on a set of homologous chromosomes, sister chromatids. During meiosis these two copies will separate, so the gametes receive one copy of each gene. Using a monohybrid cross, we can demonstrate random segregation. First we have a parental cross between two individuals that differ in the genotype of one gene. The offspring of the parental generation is called the F1 or first filial generation. The F1 generation can interbreed or self-fertilize to produce the F2 or second filial generation.
Parental Cross: P 1 x P 2 F1 Generation: F1 F1 Cross: F1 a x F1 b F2 Generation: F2 And so on, etc. For our example, we will examine the fruit fly, Drosophila. Like humans, Drosophila is diploid and has two homologous sets of chromosomes. In this case all genes exist in two copies. When writing to genotype for Drosophila, the homologous alleles are separated by a slash or horizontal line. Writing the genotype (for diploid organisms): If the trait we are looking at is Radish, then we can use a letter to denote the gene. For Radish there are two alleles, a dominant and recessive allele. Typically the dominant allele is denoted by a capital letter, i.e. R, and the recessive allele is denoted by a lowercase letter, i.e. r. For this example, we can have three possible genotypes for the different combinations of the alleles to create the diploid genotype. Homozygous dominant: RR Homozygous recessive: rr Heterozygous: Rr Homozygous refers to both alleles being the same, while heterozygous refers to having different alleles. It is possible to have more than two options, but for simplicity this course will stick to two alleles, a dominant and recessive.
Once we have the genotype, we can determine the phenotype. The dominant allele will be expressed over the recessive allele. In the case of Radish, RR and Rr will have the dominant phenotype, while rr will have the recessive phenotype. This concept can be applied to Drosophila. In this case, the variable mutant allele to the wild type allele can be dominant or recessive. The dominant mutant alleles will be written in capital letters and the recessive mutant alleles will be written in lower case letters. The wild type allele is written as + and is considered the normal phenotype that occurs in nature. Written genotypes will be written as the two alleles listed with a slash in between, e.g. +/B. Drosophila inheritance patterns and genotypes Phenotype Abbreviation Genotypes Wild type + +/+ or +/? Eye shape Bar B B/B or B/+ Eyeless (reduced) ey ey/ey Lobe L L/L or L/+ Eye color Sepia se se/se Red (wild type) + +/+ White X-linked w Female X W /X W Male X W /Y Wings Apterous (wingless) ap ap/ap Vestigial vg vg/vg Curly Cy Cy/Cy (lethal) or Cy/+ Dumpy dpy dpy/dpy Bristles singed sn sn/sn spineless ss ss/ss Body color ebony e e/e yellow y y/y
Monohybrid Cross (one gene) Parental cross: male B/+ x female B/+ male/female B + B BB B/+ + B/+ +/+ What is the genotypic ratio? What is the phenotypic ratio? 1. Parental cross: male e/e x female e/+ male/female What is the genotypic ratio? What is the phenotypic ratio?
2. Parental cross: male L/+ x female L/+ male/female What is the genotypic ratio? What is the phenotypic ratio? 3. Parental cross: male Cy/+ x female Cy/+ male/female Genotypic ratio: Why is this ratio not 1:2:1? Phenotypic ratio:
Law of Independent Assortment: During meiosis, the alleles of two different genes will separate independently of one another, unless genes are linked. Linked genes are located on the same chromosome. Those genes that are on different chromosomes will follow the principle of independent assortment. Using a dihybrid cross, we can demonstrate independent assortment and the ratios of potential offspring in a cross. Dihybrid Cross 4. Parental cross: female w/+; vg/ x male w/y: vg/+; +/+ In this case we have a normal phenotype female that is carrying a white eye (w) and vestigial wing (vg) recessive alleles crossed with a white-eyed male with normal wings, but carrying the vg allele. Female gametes: Male gametes: male/female w;vg w;+ +;vg + ;+ w;vg w/w;vg/vg w/w;+/vg +/w;vg/vg +/w;+/vg w;+ w/w;vg/+ w/w;+/+ +/w;vg/+ +/w;+/+ Y;vg w/y;vg/vg w/y;+/vg +/Y;vg/vg +/Y;+/vg Y;+ w/y;vg/+ w/y;+/+ +/Y;vg/+ +/Y;+/+ Genotypic Ratio: Phenotypic Ratio:
5. Parental Cross Female Cy/+; e/e x Male +/+; +/e Female gametes: Male gametes: Genotypic Ratio: Phenotypic Ratio:
6. F1: Female vg/vg; s/s x male +/vg; s/+ F2 genotypic ratio: F2 phenotypic ratio:
II. Polygenic inheritance Objectives: To understand the properties and patterns of polygenic inheritance. To apply a T-test to a data set and analyze polygenic inheritance. The rules of Mendelian inheritance are most often used to explain how single gene traits are inherited and their predicted ratios. In other cases, multiple genes will contribute to an observed phenotype known as Polygenic Inheritance. While Mendelian inheritance explains dominant and recessive alleles and their genotypes as discrete, polygenic inheritance shows continuous variation. Polygenic traits are influenced by multiple genes are therefor have a spectrum of traits. Polygenic traits are known to be multifactorial as expression is often affected by environmental factors. Assumptions of polygenic inheritance model: - Trait is controlled by many loci - Alleles lack dominance, but active alleles contribute to the phenotype - Genes follow Mendelian principles of random segregation and independent assortment For this lab, we are going to analyze our own phenotypes: In humans, hair color, skin tone, eye color, and height are all polygenic-inherited traits that we can use to analyze populations. Eye color is influenced by up to 16 genes. The color is determined by the amount of melanin in the front of the iris. In particular, two genes on chromosome 15 that contribute to eye color are OCA2 and HERC2. For example, we can look at these two genes and their influence on our inherited eye color. For example, a light brown eyed individual would have a genotype of BbGg. The allele for black (B) is dominant to blue (b). And the dark hue (G) is dominant and produces green color, while the light hue (g) is recessive. All dominant alleles produce black eye color. Two-three dominant alleles produces brown eye color, one dominant green, and all recessive is blue. There are five phenotypes and nine genotypes possible. Black: BBGG Dark Brown: BBGg, BbGG Light Brown/Dark Hazel: BbGg, BBgg, bbgg Green/Light Hazel: Bbgg, bbgg Blue: bbgg
7. Write down your phenotype and possible genotypes. 8. Imagine all individuals at your table group are siblings from the same parents. Based on each individual s eye color, work backwards to figure out the corresponding genotypes and the parental cross genotypes/phenotypes using a dihybrid cross.
9. Height is also a polygenic inherited trait. For the entire class, gather data on each individual s height in cm and sex. # Height (cm) Sex (M/F) Averages Female Male Range 10. Figure out the mean (averages) for the entire class and males and females. What is the shortest height? What is the tallest height?
11. Create a histogram for the data. The x-axis will be the height and the y-axis will be the frequency or number of individuals. This is to show the relative frequency of height in a graphic and informative manner. Understanding the spread of the data, where the heights fall and the range from shortest to tallest will help determine the binning, or interval for each bin that will hold the frequency for the number of individuals within that height range. 12. Save this data for further analysis using statistical inference.