chapter 4: modern genetics section 1: human inheritance
TRANSCRIPT
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Chapter 4: Modern Genetics
Section 1: Human Inheritance
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Traits Controlled by Single Genes
• Many human traits are controlled by a single gene (one dominant allele, one recessive allele)– These traits have two distinct phenotypes– Ex: Widow’s peak, smile dimples
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Multiple Alleles
• Some human traits are controlled by a single gene that has more than two alleles
• Multiple alleles – 3 or more forms of a gene that code for a single trait
• Even if a gene has multiple alleles, each person can only carry 2
• Ex: Blood type – A, B, AB, or O
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Traits Controlled by Many Genes
• Some human traits show a large number of phenotypes because the traits are controlled by many genes which act together to produce the one trait
• At least 4 genes control height• At least 3 genes control skin color• This is why there are so many variations in
these two traits
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The Effect of Environment
• Effects of genes are often changed by the environment
• Ex: height – diet poor in nutrients can prevent a person from growing to their full height
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Male or Female?
• The sex chromosomes are the only pair of chromosomes that don’t always match
• XX = female, XY = male
• All eggs from the mother are X
• Sperm from the father can be either X or Y– If an X sperm matches with the X egg, the baby
will be a girl– If a Y sperm matches with the X egg, the baby
will be a boy
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Sex-Linked Genes
• Some traits occur more often in one sex than the other because they are carried on the sex chromosomes
• Sex-linked genes – Genes on the X and Y chromosome whose alleles are passed from parent to child on a sex chromosome
• A dominant female will mask a recessive allele
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Sex-Linked Genes
• Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele
• Ex: Colorblindness• Carrier – a person who has one recessive
allele for a trait and one dominant allele– Does not exhibit the trait, but can pass it on– Only females can be carriers for sex-linked
traits
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Pedigrees
• Pedigree – a chart or “family tree” that tracks which members of a family have a particular trait
• Different symbols represent males and females and different shadings represent the presence of a trait