Chapter 12- 4: Pedigrees, Sex-linked traits, and Test Cross
Bellringer
Since the dawn of agriculture, people have used selective breeding to improve crops and domestic animals. Modern applications of Mendelian genetics and gene technology have resulted in major changes in crops and animals.
List some examples of selective breeding in domestic animals or crops that you know of. Explain how you might go about selecting for a particular trait.
Using a Pedigree
•A pedigree is a diagram that shows how a trait is inherited over several generations of a family.
•Pedigrees can be used to help a family understand a genetic disorder.
•A genetic disorder is a disease or disorder that can be inherited.
•A pedigree can help answer questions about three aspects of inheritance: sex linkage, dominance, and heterozygosity.
Pedigree Example
Pedigree symbols
Female – round
Male – square
Filled shape – has disease or trait
Half-filled shape – heterozygote or carrier
Horizontal line – mate
Vertical line - offspring
Dominant Pedigree - Characteristics of a Dominant Pedigree
–Two affected individuals may have unaffected children
–Every affected individual has at least one affected parent
–Affected individuals have a 50% chance of unaffected children if they mate with unaffected individuals
•Every affected individual has at least one affected parent
•Affected individuals have a 50% chance of unaffected children if they mate with unaffected individuals
Recessive Pedigree - Characteristics of Recessive Pedigree
–An individual who is affected may have parents who are not affected
–All of the children of two affected individuals will be affected
Recessive Pedigree
•An individual who is affected may have parents who are not affected
•All of the children of two affected individuals will be affected
Dominant or Recessive?
Recessive – affected individuals may have two unaffected parents
Dominant or Recessive?
Dominant – every affected individual has an affected parent
NOT Recessive – 2 affected parents have some unaffected children
Pedigree and Mutations Objectives
•How do you identify dominant or recessive inheritance of a genetic trait from a pedigree? Affected Parents
Using a Pedigree
•The sex chromosomes, X and Y, carry genes for many characters other than gender.
•A sex-linked gene is located on either an X or a Y chromosome.
•Traits that are not expressed equally in both sexes are commonly called sex-linked traits.
•Colorblindness is an example of a sex-linked trait that is expressed more in males than in females.
Sex-Linked Genes and Traits
–Found on the X chromosome.
–Males are XY. Females are XX.
–Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait.
–There is no additional X-chromosome allele to mask the recessive trait
Sex-linked Punnett Square
•Mom is a Carrier
•Dad is normal
•What is the likelihood that male children have the disease?
•What is the likelihood that female children are carriers?
Color Blindness is Sex Linked
•Half-filled circle is carrier
•Filled square is affected individual
•Why do only the males have the disease?
•Why are only daughter’s carriers?
Sex-linked Pedigree
Why is the daughter (III-5) affected with color blindness?
Draw a Punnett square for her parents.
Using a Pedigree, continued
•Pedigrees follow the same pattern of inheritance as Punnett squares.
• A person with a trait that is autosomal and dominant will show the trait with one dominant allele.
•If a person has a recessive trait and only one recessive allele, he or she will not show the trait but may pass it on.
•If a person is homozygous recessive, his or her phenotype will show the recessive trait.
•A recessive trait in a child shows that both parents were heterozygous carriers of that recessive allele.
Test Cross
•A test cross will determine the genotype of an individual with a dominant phenotype.
•A testcross is done by crossing a dominant phenotype and unknown genotype with a homozygous recessive individual.
•A heterozygote with a dominant phenotype will produce offspring with a recessive phenotype .
•A homozygous dominant genotype and phenotype will produce offspring with only the dominant phenotype.
Test Cross
100% dominant phenotype 50% dominant phenotype
= dominant genotype = heterozygous genotype
Summary
•A pedigree can help answer questions about three aspects of inheritance: sex linkage, dominance, and heterozygosity.
•Sex-linked traits are exhibited more by males because they do not have a second X chromosome to mask the recessive gene.
•A test cross can determine the genotype of an individual with a dominant phenotype.