Chapter 6 BHeredity Punnett Squares What are Punnett
Chapter 6 BHeredity
Punnett Squares What are Punnett squares? • Used to represent genetic crosses. • A grid that is divided into 4 boxes and uses letters to represent alleles
Punnett Squares Remember tall pea plants and short pea plants?
Punnett Squares Parent generation is called the “P generation” 2 nd generation is called “F 1 generation” 3 rd generation is called “F 2 generation”
Punnett Squares So F 1 has a 4/4 chance of being Tt What about F 2?
Punnett Squares F 2 had a ¼ (25%) chance of being TT, 2/4 (50%) chance of being Tt, and a ¼ (25%) chance of being tt
Pedigree Circle- girl (XX) Square- boy (XY) Dark color- have a disease/ trait ◦ Need both genes to have the disease/ trait ◦ Light color- dominant phenotype ◦ Dark color- recessive
Variations of Mendel’s Theories Incomplete Dominance◦ 2 different alleles for the same trait are both shown ◦ 2 traits blend
Variations of Mendel’s Theories Codominance◦ Both alleles for the trait are dominant and both are shown in hybrid offspring ◦ NOT blended ◦ Black chicken + white chicken = grey? ◦ NOPE! ◦ Black AND white chicken!
Variations of Mendel’s Theories Polygenic Inheritance◦ 2 or more genes are responsible for a single trait ◦ Skin color, height, hair color, intelligence, and more
Variations of Mendel’s Theories Multiple Alleles◦ More than 2 alleles for a gene ◦ Example: Blood type! ◦ I A, I B , I i ◦ You can be type A- IAIA or IAi ◦ You can be type B- IBIB or IBi ◦ You can be type AB- IAIB ◦ Or you can be type O- ii
Sex Chromosomes and Inheritance People have 23 pairs of chromosomes ◦ 23 rd pair (last) contains the sex chromosomes ◦ X chromosome ◦ Y chromosome Females have 2 X chromosomes (XX) Males have 1 X and 1 Y chromosome (XY)
Sex Chromosomes and Inheritance Why is this important? ◦ Some traits are on the X and/or Y chromosome ◦ Sex-linked traits ◦ (Most are X linked since Y chromosome is small) ◦ Example: red-green colorblindness, hemophilia (bleeding easily), baldness ◦ When mom has the allele but does not show it, she is a carrier
Sex Chromosomes and Inheritance Red-Green Colorblindness: ◦ X-linked trait: XG means normal (dominant), Xg means colorblind (recessive)
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