Ch 5 review Flashcards
List the four main principles of the Mendelian
model of inheritance
- Genes can exist in different forms (alleles)
- For each gene, an individual organism inherits two alleles, one from each parent.
(same=homo, diff=hetero) - An allele may be expressed (appear) as a trait or else hidden in a heterozygous individual (dom/rec)
- Each gamete (sperm or egg) carries only one allele for each hereditary trait due to the separation of allele pairs when the gametes form. Allele pairs are re-formed in fertilization.
State the law of independent assortment.
-the alleles of two (or more) different genes get sorted into gametes independently of one another
-in other words, the allele a gamete receives for one gene does not influence the allele received for another gene.
What is genetic linkage? How does linkage
affect the inheritance of certain traits?
Genes that are sufficiently close together on a chromosome will tend to “stick together,” and the versions (alleles) of those genes that are together on a chromosome will tend to be inherited as a pair more often than not.
This phenomenon is called genetic linkage.
If two genes are inherited together more than 50% of the time, this is evidence that they are linked on the same chromosome. The closer together the genes are, the more frequently they will be inherited together.
What is the difference between multiple alleles
and polygenic inheritance?
In multiple alleles, more than two alleles in a single gene affect the phenotype. (e.g. ABO blood type)
In polygenic traits, a single phenotype is affected by more than one gene. (eye colour, height, weight)
Give an example of a sex-liked disorder.
Describe how this disorder is inherited.
Hemophilia: X-linked recessive disorder
Inherited:
-in males, one copy of the affected gene on their X chromosome passes the disorder to them.
(A characteristic of X-linked inheritance is that fathers CAN’T pass X-linked traits to their sons)
-in females, 2 copies of the affected gene on their X chromosomes passes the disorder to them.
Differentiate between codominance and
incomplete dominance, using an example
for each.
Codominance: both alleles are expressed at the SAME time(offspring= equal mix of both parents)
-e.g. roan cow(white+red= RW(roan))
Incomplete: intermediate “in-between” phenotype falls between the 2 parental phenotypes.
-e.g. snapdragons (red+white=Rr(pink))
How did Mendel explain why offspring inherit
traits from previous generations?
-Law of segregation:
Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.
Explain the concept of independent
assortment
the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.
E.g. -fur colour (black or white)
-eye colour (green or red)
Two-hybrid rabbits are crossed. Both the rabbits have a genotype BbGg. Before breeding each rabbit produced gametes. During this, the alleles are separated and the copy of each chromosome is assigned to different gamete. That means, regardless of the parental phenotype, the baby rabbits inherit different combinations of the traits. Alternatively, a baby rabbit can have a genotype Bbgg.
What evidence from Mendel’s experiments
demonstrated independent assortment?
In the dihybrid cross, he chose round-yellow seed and wrinkled green seed and crossed them. He obtained only round yellow seeds in the F1 generation. Later, self-pollination of F1 progeny gave four different combinations of seeds in the F2 generation. He obtained round-yellow, wrinkled-yellow, round green and wrinkled green seeds in the phenotypic ratio 9:3:3:1.
The phenotypic ratio 3:1 of yellow: green colour and the ratio 3:1 of the round: wrinkled seed shape during monohybrid cross was retained in the dihybrid cross as well. Thus, he concluded that characters are distributed independently and inherited independently. Based on this observation, he developed his third law – Law of Independent Assortment.
The fruit fly phenotype depicted in the following
photograph helped Thomas Morgan with an
important genetic discovery. Explain what that
discovery was and describe the observations that
led Morgan to his conclusion
-Thomas Hunt Morgan, who studied fruit flies, provided the first strong confirmation of the chromosome theory.
-Morgan discovered a mutation that affected fly eye color.
-He observed that the mutation was inherited differently by male and female flies.
-Based on the inheritance pattern, Morgan concluded that the eye color gene must be located on the X chromosome
=sex-linked traits
If a trait has a continuous range of phenotypes,
what is the most likely mode of inheritance?
Explain
Polygenic trait
-A polygenic trait is one whose phenotype is influenced by more than one gene
Why are more males than females affected by
sex-linked disorders?
-males=hemizygous (only need one gene=affected)
-females (very rare- need 2 genes= affected)
How does studying crossing over of
chromosomes help us identify gene loci?
-crossing over (meiosis) occurs= gene loci far apart
-genetic linkage(non-meiosis) occurs= gene loci close together
Suggest two or more reasons that fruit flies are
good model organisms for studying genetics
-produce large numbers of offspring very quickly
-Drosophila have ‘polytene’ chromosomes, which means that they are oversized and have barcode-like banding patterns of light and dark. During early Drosophila research scientists could therefore easily identify chromosomal rearrangements and deletions under the microscope
-relatively straightforward to mutate (disrupt or alter) fruit fly genes because they have a smaller genome
How can the environment influence the
expression of a particular trait? Explain using
an example.
-In genetics, “environment” refers to the conditions in which a cell or organism lives. For example, although a tree’s genotype does not change throughout its lifetime, the shape, size, and colour of its leaves greatly depend on the tree’s exposure to sunlight and humidity
e.g. The Siamese cat is covered in creamy white fur, except on its ears, face, feet, and tail.
-The pattern is due to a mutation in a gene for pigment production.
-As a result of the mutation, the enzyme responsible for black fur is sensitive to heat.
The enzyme is active only in the cooler temperatures found at the cat’s extremities, giving the Siamese cat its characteristic appearance
