Extensions of Mendelian Inheritance The previous situations describe examples of Complete Dominance. There is a lot more to inheritance of most traits than what is described above, but typically, Mendels Laws apply. Here are some extensions of Mendelian Genetics: Incomplete Dominance Incomplete dominance occurs when a trait is regulated by two alleles, but neither allele is truly dominant over the other. In incomplete dominance, heterozygotes have a different phenotype than the two different homozygous phenotypes. Generally, it is an intermediate phenotype between the two homozygous phenotypes.

Homozygous for Allele 1 = Phenotype 1 Homozygous for Allele 2 = Phenotype 2

Heterozygous = Intermediate phenotype Petunia flower color is a good example of incomplete dominance. The homozygous phenotypes are red (R1R1) and white (R2R2). The heterozygous phenotype is pink (R1R2), a blending of the two homozygous phenotypes. Lets perform a monohybrid cross. In this cross, a pink individual (R1R2) was mated with a pink (R1R2) individual, what possible offspring could be produced?

Predict the outcome of a monohybrid cross of a trait expressing incomplete dominance: R1R2 with R1R2 What is the genotypic ratio of the F1 generation? What is the phenotypic ratio of the F1 generation?

Codominance Codominance is another type of inheritance that involves the full expression of both phenotypes in the heterozygous individual.

Homozygous for Allele 1 = Phenotype 1 Homozygous for Allele 2 = Phenotype 2

Heterozygous = Both phenotypes are expressed

Coat color in shorthorn cattle exhibits codominance. The homozygous phenotypes are red (CRCR) and white (CWCW). The heterozygous phenotype is roan (CRCW). Roan cattle do not express a third hair color (this would be incomplete dominance), instead they have both red hairs and white hairs that are mixed together in the coat, giving them an overall appearance (phenotype) of roan coloration.

Lets perform a monohybrid cross. In this cross, a roan cow (CRCW) was mated with a roan bull (CRCW), what possible offspring could be produced? Predict the outcome of a monohybrid cross of a codominant trait: CRCW with CRCW

What is the genotypic ratio of the F1 generation? What is the phenotypic ratio of the F1 generation?

Sex-Linkage In primates, such as humans, and many other species, sex (male or female) is determined by the two sex chromosomes. The sex chromosomes have alleles for traits just like other chromosomes (called autosomes). Genes on the sex chromosomes are referred to as sex-linked genes. The X chromosome is large and contains genes for many different traits. The Y chromosome lacks many genes that are found on the X chromosome. This is due to two main factors: the Y chromosome is much smaller than the X chromosome, and the Y chromosome contains unique male genes. Most importantly it contains the SRY gene region, which when present, sets in place a series of developmental events that make an individual male. Therefore at these two spots: Females are XX Males are XY Because males only have one X and females have two, the likelihood of inheriting a gene on the sex chromosomes, a sex-linked gene, is different in males and females In order to get a better understanding of sex-linked traits, we will perform several monohybrid crosses. In these crosses, pay special attention to the mothers and to the fathers phenotype and to which offspring (sons and daughters) that phenotype is passed.

Colorblindness is an example of a recessive allele that is found only on the X-chromosome. Therefore it is sex-linked, but more specifically it is X-linked. What would happen if we cross a male with normal vision (XNY) with a female carrier (meaning that she is heterozygous and has the allele for colorblindness, but it is masked by the dominant allele, so she has normal vision) (XNXN)? Lets plug this into a Punnett Square and find out. The female has two Xs, one with the

recessive trait (n) and one with the dominant trait (N). The male only has one X, which has the normal allele (N), and a Y, which has no alleles for this gene. We get the following set up: When can predict the outcomes by moving both the sex chromosomes and the alleles for colorblindness: We now have two phenotypes to observe, in the following ratios:

1:1 male to female 3:1 normal vision to colorblind.

But notice that the colorblind offspring will only be male. This is why we see this trait in more males than females. Half the females will be carriers and half will be homozygous for normal vision.

Female XN Xn

Male XN

Y

Female XN Xn

Male XN XN XN XN Xn

Y XNY XnY

1

2

Try some of these on your own. 1. Preform a sex-linked cross to determine what possible offspring could be produced.by a Normal

Vision Male (XNY) crossed with Colorblind Female (XnXn).

What is the genotypic ratio of the F1 generation? What is the phenotypic ratio of the F1 generation?

2. Preform a sex-linked cross to determine what possible offspring could be produced.by a Colorblind

Male (XnY) crossed with Normal Vision Female (XNXN).

What is the genotypic ratio of the F1 generation? What is the phenotypic ratio of the F1 generation? 3. Preform a sex-linked cross to determine what possible offspring could be produced.by a Colorblind

Male (XnY) crossed with Normal Vision Female (XNXn).

What is the genotypic ratio of the F1 generation? What is the phenotypic ratio of the F1 generation?

Pedigree Analysis A pedigree is a chart that visually displays the phenotypes of a given trait for an organism and its ancestors. Each row represents a generation: I = grandparents, II = parents, III = children. Squares represent males and circles represent females. We may or may not have measured their genotypes, but we know their phenotypes. Solid shapes are individuals that display that trait (they have the phenotype of interest). Open shapes do not have the phenotype, but they may be carriers. Based on the pattern of inheritance, we might be able to detect if the trait of interest is dominant or recessive and the genotypes of the individuals.

a. b.

1. Is this trait dominant or recessive? How can you tell?

2. What are the genotypes of the two marked individuals? a. b.

Pedigree Exercise You are a genetic counselor; translate the following passage into a pedigree. My name is Jack, and I have normal body coloration. My older sister has albino coloration. My dad and his older brother are normal; however, my dads younger sister is albino. My paternal grandfather is albino, and my paternal grandmother is normal. My mom and her 2 sisters are normal. My maternal grandfather and grandmother are normal. 3. Draw Jacks pedigree on a separate sheet of paper.

4. What is the chance that Jack is a non-symptomatic carrier for the albino trait?

Extra practice: 5. What is the phenotype and genotype ratio of a cross between a brown male rat XbY with black female

rat XBXb ?

6. What is the phenotype and genotype ratio of a cross between a green bodied- short wing dragonfly (bbll) and a blue bodied- long-winged dragonfly (BbLl) ?

7. Using a chi-square formula, determine if the data supports the 9:3:3:1 ratio hypothesis. Be sure to show your work and report the 2 value and p-value.

Given: You perform a cross of 2 heterozygous frogs. At least you think the frogs are heterozygous.

Your parent frogs are Red Eyed and Long-Toed (RrLl). If it really is a cross of heterozygotes, you predict the standard 9:3:3:1 ratio, as seen in dihybrid crosses. Your frogs reproduce and produce the following offspring:

102 Blue-eye short-toed, 312 Blue-eye long-toed, 305 Red-eyed short-toed, 905 Red-eye long-toed. What are your results and conclusions? 8. 8. Using a chi-square formula, determine if the data supports the 9:3:3:1 ratio hypothesis. Be sure to

show your work and report the 2 value and p-value. Given: You perform a cross of 2 heterozygous fruit flies. At least you think the flies are

heterozygous. Your parent flies are Red Eyed and Hairy (RrHh). If it really is a cross of heterozygotes, you predict the standard 9:3:3:1 ratio, as seen in dihybrid crosses. Your flies reproduce and produce the following offspring:

147 Red Hairless Flies, 442 Red Hairy Flies, 55 White Hairless flies, and 151 White Hairy flies. What are your results and conclusions?