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Incomplete Dominance • One allele is not completely dominant over another Incomplete Dominance • A flower has genes for white petals (w) and red petals (r). When you mix white flowers with red flowers you get pink flowers. How is this possible? The w gene does not dominate over the r gene and vice versa. Both genes express themselves equally so you get a pink color in the petals. w w r wr wr r wr wr Codominance • Both alleles contribute to the phenotype Polygenetic Traits • Traits controlled by two or more genes Incomplete and Co-Dominance Punnett Squares Incomplete Dominance • RR x WW where RR is red and WW is white R W W R Incomplete Dominance • RR x WW where RR is red and WW is white • Offspring RW are pink R R W RW RW W RW RW Co-Dominance • RR x WW where RR is red and WW is white R W W R Co-Dominance • RR x WW where RR is red and WW is white • Offspring RW show red and white: ex. Red Roan Horse R R W RW RW W RW RW Karyotypes and Sex-Linked Traits • There are traits that are controlled by one gene with 2 alleles. Often, one is dominant and the other is recessive • Example: widow’s peaks and dimples. Some traits are controlled by a gene with multiple alleles – 3 or more for a single trait. For example: blood types and skin color in humans. • There are 44 chromosomes that we call autosomal chromosomes. • However, there are 2 chromosomes that determine our sex and we call them sex chromosomes. • These 46 chromosomes all carrier genes on them that determine our traits. • Out of our 23 pairs of chromosomes, 1 pair is the sex chromosomes (X and Y). • Female = XX • Male = XY • Question: What is the probability that your parents will have a boy or girl? • XY (dad) x XX (mom) X X XX Y XY Phenotype: 50% boy 50% girl X Genotype: XX XY 50% XX 50% XY Question? If my parents have 5 boys in a row, what is the chance they will have a girl the next time? 50% Sex-linked gene: • Some traits are carried on the sex chromosomes. Genes on the X or Y chromosomes are sex-linked genes. • These traits are passes on from parent to child. Sex- linked genes can be recessive or dominant. • MALES are more likely to have a sex-linked trait because they only have ONE X and Y. The allele is USUALLY on the X chromosome. • Ex. colorblindness, hemophilia, hairy ears, muscular dystrophy Are you colorblind? What numbers do you see? Carrier – person who has one recessive allele and one dominant allele for a trait or heterozygous for that trait (only women can be carriers). Example Hemophiliac carrier XHXh Colorblind carrier XBXb Sex linked Punnett Squares: Question: What is the probability that a carrier female and a colorblind male will have a girl who is colorblind (b = colorblind, B = normal)? Xb Y Phenotype: XB XBXb XBY 25% normal boy 25% colorblind boy 25% normal girl Xb 25% colorblind girl XbXb XbY Try this one on your own Question: What is the probability that a homozygous (normal vision) female and a colorblind male will have a girl who is colorblind (b = colorblind, B = normal)? Parents: XBXB x Xb X bY Y Phenotype: XB XBXb XBY XB XBXb XBY 50% normal girls 50% normal boys
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