BLG 143 Study Guide - Final Guide: Y Chromosome, Dna Replication, Zygosity
13.6 Applying Mendel’s Rules to Humans
To understand the transmission of human traits, investigators have to analyze human
genotypes and phenotypes that already exist. They construct a pedigree/family tree of
affected individuals. A pedigree records the genetic relationships among the individuals in
a family along with each person’s sex and phenotype with respect to the trait in question.
If the trait is due to a single gene, pedigree may reveal if trait is due to dominant or
recessive allele and whether the gene is located on a sex chromosome or autosome.
Biologists assume the simplest case, that a single autosomal gene is involved and alleles
present in population have simple dominant-recessive relationship.
In analyzing inheritance of traits, distinguish conditions that must be met when a particular
pattern of inheritance occurs vs. conditions that’re likely to be met.
If a phenotype is due to an autosomal recessive allele, then:
• Individuals with the trait must be homozygous
• Where the parents of an affect individual don’t have the trait, the parents are likely
to be heterozygous for that trait
Heterozygous individuals who carry a recessive allele for an inherited disease are carriers
of the disease.
In general, a recessive phenotype should show up in offspring only when both parents have
the recessive allele and pass it on to their offspring. A recessive allele produces a given
phenotype only when the individual is homozygous for it.
Human Traits as Autosomal or Sex-Linked?
If males are much more likely to have the trait in question than females, then the allele is
likely to be recessive and found on the X chromosome.
Since so few genes occur on the Y chromosome, Y-linkage is rare.
Hemophilia is caused by a defect in an important blood-clotting factor. It’s an X-linked trait
resulting from recessive allele.
The appearance of an X-linked recessive trait skips a generation in a pedigree.
A good indicator of a X-linked dominant trait is a pedigree in which an affected male has all
affected daughters but no affected sons.
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Chapter 14 – DNA and the Gene: Synthesis and Repair
➢ Genes are made up of DNA
➢ When DNA is being copied, each strand of the double helix serves as the template for
the synthesis of a complementary strand
➢ DNA synthesis occurs in the 5’ to 3’ direction only and requires a large suite of
specialized enzymes
➢ The leading strand in synthesized continuously, but the lagging strand is
synthesized as a series of fragments that are then linked together
➢ Specialized enzymes repair damages to DNA and fix mistakes in DNA synthesis
➢ If these enzymes are defective, the mutation rate increases
14.1 What are Genes Made Of?
It had been known that chromosomes are a complex of DNA and proteins, but didn’t know
whether genes were made of DNA or protein. The general consensus was that genes were
comprised of proteins because of relative complexity, variability structure and function.
DNA was known to be composed of 4 types of deoxyribonucleotides. Thought to be simple
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