TOPIC 10: Genetics
10.1 Meiosis
10.1.1: Describe the behavior of the chromosomes in the phases of meiosis
 Prophase I - homologous chromosomes join together to form a tetrad in a process called synapsis.
 Metaphase I - homologous pairs line up along the central axis of the cell.
 Anaphase I - homologous chromosomes are separated and pulled toward the nearest pole; sister
chromatids move together.
 Telophase I - the chromosomes have formed two groups; one group at each pole.
 At cytokinesis, when the diploid parent cell divides, each daughter cell has only 1 of each kind of
chromosome.





Prophase II - the chromosomes condense and become visible in a light microscope.
Metaphase II - the chromosomes line up along the central axis (equator) of the cell.
Anaphase II - the centromeres are broken and the sister chromatids are pulled to opposite poles.
Telophase II - the chromatids (now chromosomes) group at the poles.
Cytokinesis produces four gametes.
10.1.2: Outline the formation of chiasmata in the process of crossing over
 Homologous chromosomes pair together, during Prophase 1 of meiosis, in a process called synapsis.
 During synapsis, X-shaped structures (called chiasmata) can form between non-sister chromatids.
 On average, 1-3 chiasmata form per homologous pair.
 At each chiasma, the maternal DNA can be swapped with the paternal DNA in a process called
crossing over.
 Crossing over produces new combinations of alleles, called recombinants.
 The process by which offspring get new combinations of alleles, which differ from the parents, is
called recombination.
1
10.1.3: Explain how meiosis results in an effectively infinite genetic variety in gametes
 Meiosis 1 is responsible for genetic variation because it is the process that separates allele pairs (by
independent assortment).
 Independent assortment refers to the random separation of chromosomes, which means that either
chromosome of a homologous pair may combine with either chromosome of another homologous
pair.
 Random assortment results in 2 to the power of n possible combinations of chromosomes in the
gametes of one parent. Therefore, a man can produce 2 to the power of 23 (8,388,608) types of
sperm.
 In addition to independent assortment, there are other sources of genetic variation, including:
crossing-over at prophase 1, and random chance of which sperm fertilizes which egg.
10.1.4: Outline Mendel’s law of independent assortment
 Mendel's Law of Independent Assortment states that: during gamete formation, the two ‘factors’ of
each characteristic separate independently of the other factors. Therefore, the inheritance of one
characteristic does not influence the inheritance of other characteristics.
10.1.5: Explain the relationship between Mendel’s law of independent assortment and meiosis
 Meiosis 1 is responsible for genetic variation because it is the process that separates allele pairs (by
independent assortment).
 Independent assortment refers to the random separation of chromosomes, which means that either
chromosome of a homologous pair may combine with either chromosome of another homologous
pair.
 Independent assortment can result in F2 offspring with combinations of alleles that differ from their
parents are called recombinants.
 Mendel developed his Law of Independent Assortment after performing a dihybrid cross with truebreeding parents (YYRR and yyrr).
 All the F1 offspring had yellow and round seeds (YyRr).
 When Mendel crossed the F1 offspring he found that on average:
 9/16 of the F2 offspring had yellow and round seeds
 3/16 had green and round seeds
 3/16 had yellow and wrinkled seeds, and
 1/16 had green and wrinkled seeds.
 This outcome matched the expected outcome that can be produced using a Punnet grid.
10.2 Dihybrid crosses and gene linkage
10.2.1: Calculate and predict the genotypic and phenotypic
ratio of offspring of dihybrid crosses
 Mendel performed a dihybrid cross with true-breeding
parents (YYRR and yyrr).
 All the F1 offspring had yellow and round seeds (YyRr).
 When Mendel crossed the F1 offspring he found that on
average:
 9/16 of the F2 offspring had yellow and round seeds
 3/16 had green and round seeds
 3/16 had yellow and wrinkled seeds, and
 1/16 had green and wrinkled seeds
2

This phenotypic ratio of 9:3:3:1 matched the expected outcome that can be produced using a
Punnet grid.
10.2.2: Distinguish between autosomes and sex chromosomes
 There are 23 pairs of homologous chromosomes in humans. One of the homologous pairs is the sex
chromosomes. The remaining 22 pairs of homologous chromosomes are called autosomal
chromosomes.
 In females, the two sex chromosomes are homologous (XX): they are large and they contain more
than 300 different genes.
 In males the two sex chromosomes are not homologous (XY): the Y is small and has only a few
genes.
10.2.3: Explain how crossing over of a homologous pair in prophase I
can result in an exchange of alleles
 During Prophase 1, portions of two homologous chromosomes can
overlap portions of their DNA.
 The overlapping regions of two chromosomes are called chiasmata.
 Homologous chromosomes can swap portions of their DNA in a
process called crossing over.
 Crossing over between can result in new combinations of alleles on
each chromosome.
 These new genotypes are different from both parents and they are
called recombinants.
 Therefore, independent assortment of alleles during Anaphase 1, is
not the only source of genetic recombination.
10.2.4: Define linkage group
Genes that are on the same pair of homologous chromosomes are called a linkage group.
10.2.5: Explain an example of a cross between two linked genes
 A Punnet grid can be used to predict phenotypic ratios of unlinked genes but can NOT be used to
predict phenotypic ratios of linked genes. This is beacause un-linked genes assort independently
during meiosis but linked genes tend to stay together throughout meiosis.
 Therefore, linked genes do not produce
the same F2 phenotypic ratio observed by
Mendel.
Example
Two fruit flies were crossed: the wings of the
female were gray and big (GgBb); the wings of
the male were black and small (ggbb). Gray is
dominant over black, and big is dominant over
small. A punnet grid (based on independent
assortment) predicted the F2 phenotypic ratio
of 1: 1: 1: 1.
3
The matings produced 2,300 offspring; however the observed values were significantly different from
the expected values.
Phenotype
Observed Values
Expected Values
Gray/Big
965
575
Black/Small
944
575
Gray/Small
206
575
Black/Big
185
575
The unexpected result is explained by crossing over in the female. When crossing over occurs, G
combines with b, and g combines with B, to make recombinant eggs. Crossing over has no effect on the
male because he is homozygous.
10.2.6: Identify which of the offspring are recombinants in a dihybrid cross involving linked genes
Complete the chart below to show the genotypes of the recombinants
10.3 Polygenic Inheritance
10.3.1: Define polygenic inheritance
Polygenic inheritance is the inheritance of a characteristic that is controlled by more than one gene.
10.3.2: Explain that polygenic inheritance can contribute to continuous variation using two examples
 Polygenic inheritance is the inheritance of a characteristic that is controlled by more than one gene.
 Polygenic inheritance can produce unexpected phenotypes (i.e., phenotypes that don’t fit with
Mendel’s expected outcomes based on independent assortment of dominant and recessive alleles).
4

Two examples of polygenic inheritance are: 1) skin colour in humans, and 2) comb shape in roosters.
Example 1: Human skin colour
 Polygenic inheritance can cause a trait to have continuous variation, which means the characteristic
does not have discrete forms and instead varies gradually between two extremes.
 Human traits that show continuous variation include weight, height and skin colour.
 In humans there are at least three genes coding for skin colour. Each skin colour gene has two
forms; one form codes for high levels of melanin production, and the other form codes for low levels
of melanin production.
 Melanin is a brown coloured pigment that protects us from the sun’s harmful UV rays. The more
melanin you have, the darker, and better protected your skin will be.
 The skin colour genes are interesting because they exhibit incomplete dominance. This means that a
person who is heterozygous for all 3 skin colour genes (Aa, Bb and Cc) has 3 units of pigment,
whereas a person homozygous for all 3 skin colour genes (AA, BB and CC) has 6 units of pigment.
 The table below shows that there are seven shades of darkness in people, based on genetics. In
reality there are far more than seven skin colour shades due to individual differences between
people as well as environmental effects such as sun tans.
5
Example 2: Rooster Combs
6
Download

Topic 10 Notes (Genetics)