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# ___ Human Heredity (Chapter 14) and Pedigree Analysis
Previously, we learned about genetics in peas, guinea pigs, and other organisms. Now we’ll learn more
about human genetics. A person may be homozygous dominant, heterozygous, or homozygous
___________________ for a certain trait. Given the allele f for earlobe attachment, a homozygous dominant
individual will have the genotype
. A heterozygous individual will have the genotype
and a
homozygous recessive individual will have the genotype
.
While genotype will influence phenotype, remember that environmental factors can also influence
how/whether a gene is expressed. (Read about western white butterflies on page 321.)
Remember that humans have
(#?) chromosomes in every cell. Of these,
pairs
are autosomes. An autosome is any chromosome except the sex chromosomes. Each person has one pair of sex
chromosomes. Females have 2 X chromosomes while males have
.
Autosomal heredity just means that the allele for the trait being discussed is located on an autosome. If a
trait is autosomal
, this means that an individual will only need one dominant
allele for the trait to be expressed in its phenotype. If a trait is autosomal
, a person
must have both recessive alleles before this trait is expressed. This is exactly the same as the genetic rules we
were following in Chapter 11.
A trait is considered sex-linked if its allele is located on a sex chromosome (usually the
chromosome). We’ll learn more about this later.
Punnett Problems (Part 1):
1. Polydactyly (having 6 digits instead of 5) is an autosomal dominant disorder. A woman who is
heterozygous for polydactyly marries a man without polydactyly.
A. What is the woman’s phenotype?
B. What is the man’s phenotype?
C. How likely are they to have a child with polydactyly?
Traits Controlled by Multiple Alleles (review of Ch 11.3, page 320; also on page 394)
You are probably used to seeing just two options for alleles, the dominant and the recessive? However,
some traits are determined by more than two alleles. One classic example is blood type. Blood type has
different alleles, often written as
,
and
, which results in 4
different phenotypes (from 6 genotypes). Note: Blood type is not just multiple-allelic. The A and B alleles are
codominant and the O allele is recessive. (What does this mean?)
2. Is it possible for a mother with blood type A and a father with blood type B have a type O child? Explain.
3. What are the possible blood types of the children of a type AB mother and a type B father?
X-Linked Inheritance
A trait is considered sex-linked if its allele is located on a sex chromosome (usually the
chromosome). The
sex chromosomes contain different genes. Genes found on the X chromosome are called
-l
genes. Genes on the Y chromosome are called -l
g
. Sex-linked traits are not autosomal.
(Of course not. Why not?)
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Punnett Problems (Part 2): Write the allele as a subscript on the sex chromosome.
4. Hemophilia is an X-linked recessive trait. If a man with hemophilia marries a carrier, how likely (%
probability) are they to have a son with hemophilia?
Phenotype of mother: not affected with hemophilia (because the allele is recessive)
Genotype of mother: XHXh
Phenotype of father: affected (and NOT homozygous recessive, because only the X chromosome
carries the allele)
Genotype of father: XhY
Likelihood of having an affected son?
5. Colorblindness is also an X-linked recessive trait. Can a colorblind man who marries a non-colorblind
woman have a son who is colorblind? Why/Why not?
6. Can a colorblind man have a colorblind daughter? Why/Why not?
Pedigree – a family tree that shows patterns of inheritance.
Pedigree 1:
The pedigree below is for a genetic disease or abnormality. We do not yet know if it is dominant or
recessive. We will determine if it is possible that the trait is autosomal recessive. If the trait were recessive, we
could use the following designations for the alleles:
A = not affected (dominant)
a = affected (a genetic disease or abnormality, recessive)
Assume for the moment that the trait is recessive (we don't know yet). The pedigree shows that three of
the individuals have the normal phenotype and one individual has the abnormal phenotype. In all pedigree
problems, it is helpful to begin with the affected individual. Write the genotype of the affected (abnormal)
individual next to her symbol in the pedigree below. If possible, write the genotypes of the three recessive
individuals next to their symbols. If you only know one of the alleles, use a "?" for the unknown letter. As you
attempt to write the genotypes, keep in mind that the pedigree may not be possible for that pattern of inheritance,
so it may not be possible to write the genotypes to correctly show the genetic relationship between parent and
child.
7. Is it possible that the pedigree above is for an autosomal recessive trait? If not, why not?
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Now assume that same pedigree is for an autosomal dominant disorder. Use the same designations you used
before. Think about what genotypes an affected individual could have. Write the possible genotypes of the
affected individual next to her symbol in the pedigree below. Continue to write the genotypes for the rest of the
family.
8. Is it possible for the pedigree above to be for an autosomal dominant trait? If not, why not?
Now try to see if this pedigree could be for an X-linked recessive pattern of inheritance.
Step 1: Write in the sex chromosomes for each family member, remembering to leave some space to
write the subscript (A or a) representing the allele.
Step 2: Complete the genotype for the affected individual. In this case, there is only one possible
genotype.
Step 3: Write in the genotype of the parents.
Step 4: Check the parents’ genotype against that of the affected child. Is it possible for the child to have
received one recessive allele from each parent? Why not?
9. Therefore, it is/is not possible for this pedigree to be for an X-linked recessive disorder
10. Still looking at the same pedigree, could it be for an X-linked dominant disorder?
11. Finally, can this pedigree be for a Y-linked disorder?
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Pedigree 2:
Possible Pattern of
Inheritance
Yes or No?
Autosomal recessive?
Autosomal dominant?
X-linked recessive?
X-linked dominant?
Y-linked?
Pedigree 3: For those looking for a challenge!
Possible Pattern of
Inheritance
Yes or No?
Autosomal recessive?
Autosomal dominant?
X-linked recessive?
X-linked dominant?
Y-linked?
To think about:
1) What can you conclude about the parents of a child that has an autosomal dominant disorder?
2) Can two individuals that have an autosomal dominant trait have unaffected children?
3) If a trait were autosomal recessive, why can’t two affected individuals have an unaffected child?
4) If a trait were autosomal recessive, what can you conclude about the unaffected parents of an affected
child?
5) What can you conclude about the children of a mother with an X-linked recessive disorder?
6) What can you conclude about father-to-son transmission of X-linked traits? Mother-to-son transmission?
7) What can you conclude about the percentage of males that would have an X-linked recessive disease
compared to the number of females?
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