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Quarter 3 HSA Review Session
Genetics, Punnett Squares and Pedigrees
GENETICS
What is genetics?
The study of heredity, which is the passing of genetic traits from parents to offspring.
Genetics Vocabulary
Vocabulary Term Definition
Dominant
Recessive
Homozygous
Heterozygous
Genotype
Phenotype
The allele that is expressed
The allele that is hidden or masked when a dominant allele is present
When the 2 alleles of a particular gene in an individual are the same (ex: PP and pp)
When the 2 alleles of a particular gene in an individual are different (ex: Pp)
An organism’s genetic makeup; set of alleles (ex: PP, Pp, pp)
The physical appearance of a trait (ex: purple flowers or white flowers)
PUNNETT SQUARES
Punnett Square A punnett square is a tool that can used to predict the results of a
genetic cross
Can be used to predict the probability of passing on autosomal or sex-linked traits to offspring
Parent 2
Parent 1
One allele One allele
One allele
One allele
Autosomal vs. Sex-Linked Traits
The majority of traits that we inherit from our parents are not dependent on our sex → autosomal traits.
Genetic traits such as tongue rolling and having a widow’s peak will appear equally in both sexes.
However, certain traits are dependent upon one’s sex. They are determined by genes that are found on the sex chromosomes → sex-linked traits.
Example: Hemophilia (X-linked recessive disorder)
Autosomal Punnett Square Problem
Tall is dominant in pea plants, and short is recessive. T=tall and t=short. What are the genotypic and phenotypic ratios for a cross of a homozygous tall plant with a heterozygous tall plant?
Homozygous tall x Heterozygous tall
T T
T
t
TT
Tt
TT
Tt
Genotypic ratio- 1 TT : 1 Tt
Phenotypic ratio- 1 tall: 0
Sex-linked Punnett Square Problem
Hemophilia is an X-linked recessive disorder. A woman has hemophilia. She marries a man, who also has hemophilia. Write the genotypic and phenotypic ratios.
Xh Xh XhXh
XhY XhY
Xh Xh
Xh
Y
Genotypic ratio- 1 Xh Xh: 1 XhY
Phenotypic ratio- 1 female with hemophilia: 1 male with hemophilia
GENETICS AND PUNNETT SQUARE
QUESTIONS
Question 1
Which of these combinations results in the expression of a recessive trait?A.two dominant allelesB.a dominant sex-linked allele and a Y chromosomeC.two recessive allelesD.a dominant allele and a recessive allele
Question 2
Which of these is an example of a heterozygous genotype?A.RrB.RRC.wrinkledD.round
Question 3
In humans the trait of having freckles (F) is dominant to not having freckles (f).Which genotype and phenotype are correctly paired?A.FF—no frecklesB.Ff—no frecklesC.Ff—frecklesD.ff—freckles
Question 4
One kind of chromosomal mutation can occur during meiosis when a pair of chromosomes that carry genes for the same trait fail to separate. Which of these represents the sex chromosomes of a male organism when this type of chromosomal mutation has occurred? A.XXYB.XXC.XYD.XXX
Question 5
Red-green color blindness affects about 7.0% of the human male population. It affects approximately 0.4% of the human female population. These data suggest that red-green color blindness is aA.dominant trait carried on the Y chromosomeB.dominant trait carried on the X chromosomeC.recessive trait carried on the Y chromosomeD.recessive trait carried on the X chromosome
Question 6
In humans, the allele for dimples (D) is dominant. The allele for not having dimples (d) is recessive. A woman (DD) and a man (Dd) have four children. Which of these is the predicted ratio of the children with dimples to the children without dimples?A.1:0B.1:1C.1:3D.3:1
PEDIGREES
What is a pedigree?
A diagram that shows the occurrence of a genetic trait in several generations of a family
Indicates a Mating
Indicates offspring
Normal male
Normal Female
Affected Male
Carrier (Female)
Affected Female
Interpreting a Pedigree
1. Determine if the pedigree is autosomal or sex-linked
If the disorder occurs equally amongst men and women the disorder is autosomal.
If most of the males in the pedigree are affected by the disorder, it is sex-linked
Interpreting a Pedigree
2. Determine whether the disorder is dominant or recessive
If the disorder is dominant, only one parent of the affected child must have the disorder.
If the disorder is recessive, neither parent of the affected child has to have the disorder because they can be heterozygous.
The pedigree above traces the inheritance of dimples in a family.
1.Is the pedigree autosomal or sex-linked?
2.Is it dominant or recessive?
3.Label the genotypes of all of the individuals in the pedigree.
The pedigree above traces the inheritance of colorblindess in a family.
1.Is the pedigree autosomal or sex-linked?
2.Is it dominant or recessive?
3.Label the genotypes of all of the individuals in the pedigree.
PEDIGREE QUESTIONS
Question 7In humans, the allele for having feet with normal arches is dominant (A). The allele for flat feet is recessive (a). The pedigree below shows the occurrence of normal arches and flat feet in four generations of a family. In the pedigree, individuals are identified by the generation and Individual numbers. For example, Individual 2 in Generation I is identified as I-2.
Which of these individuals in the pedigree is a male with the genotype aa?A. Individual I-1B. Individual II-2C. Individual III-2D. Individual III-5
Question 8In humans, the allele for having feet with normal arches is dominant (A). The allele for flat feet is recessive (a). The pedigree below shows the occurrence of normal arches and flat feet in four generations of a family. In the pedigree, individuals are identified by the generation and Individual numbers. For example, Individual 2 in Generation I is identified as I-2.
Individuals III-6 and III-7 have two children and are expecting a third child. Their two children have flat feet. What is the chance that the third child will have normal arches?A.25%B.50%C.75%D.100%
Question 9In humans, the allele for having feet with normal arches is dominant (A). The allele for flat feet is recessive (a). The pedigree below shows the occurrence of normal arches and flat feet in four generations of a family. In the pedigree, individuals are identified by the generation and Individual numbers. For example, Individual 2 in Generation I is identified as I-2.
Which of these Punnett squares shows the cross between Individual II-4 and Individual II-5?