Part Four…. AS 90715 Describe the role of DNA in relation to gene expression. Determination of Phenotype. Take me to NZQA Documents relating to this standard. Contents. Monhybrid and Dihybrid crosses Incomplete/ Codominant Multiple Alleles and Lethal Genes Linked Genes Recombination
Determination of Phenotype
Take me to NZQA Documents relating to this standard
The cross of the F1 generation:
PP is homozygous dominant
pp is homozygous recessive
Pp is heterozygous
Also called “pure breeding”
The genotype is a description of the genes contained in the individual
The phenotype is a description of its physical appearance (e.g. Purple)
In bears, white ears are recessive to black
Momma bear (white)
What is the genotype of Momma Bear?
Poppa bear (black)
What genotypes could Poppa Bear have?
Poppa bear=________ or _______
This is baby bear (white eared).
What is his genotype?
Baby bear =
Can we say something more about Poppa’s genotype?
Not linked (not on the same chromosome).
In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is dominant to Pentagonal.
2 Quarks both EeTt are crossed:
In yr 10, yr 11 and yes again in yr 12 we lead you to believe that alleles were either recessive or dominate. But now that you are big grown up yr 13s – it is time we expanded you understanding!
Yeah sure some alleles are dominant and recessive
In Incomplete Dominance an intermediate phenotype is produced:
In Codominance both alleles are expressed at the same time:
It is possible to have more than 2 alleles for a particular trait.
Example – In humans the blood group is determined by 3 alleles
A common example is the ABO blood groups in humans:
O is non-functional
A forms a protein with A antigen
B forms a protein with B antigen
A and B are codominant
Lethal genes are ones that cause death in the individual. The lethal gene may be dominant or recessive.
In the heterozygous individual there may be some observed difference, e.g. Manx (tailless) cats. Even when dominant the lethal gene may be passed on if it does not have onset until after reproductive age (e.g. Huntington’s).
Linked genes are on the same chromosome.
This means that when cell division occurs the 2 genes are likely to stay together.
So where we might expect a offspring phenotype ratio of 1:1:1:1, we actually get something else.
Two genes B (Bent) and D (Dark) are linked.
For a cross between BbDd and bbdd…
Draw the gametes each could form.
Draw a punnet square for the cross.
Explain these results:
Bent Dark: Bent Light: Straight Dark: Straight Light
B and D (and b and d) are linked. The 1 Bbdd and 3 bbDd individuals are due to crossing over. The different numbers are due to random chance.
Recombination (crossing over) during meiosis increases gamete variation.
1. Homologous chromosomes line up
3. Meiotic division
2. Chiasma forms, segments swap
4. Mitotic division
The closer the genes, the more likely they stay together
Crossover value (%) =
No. of offspring
We can “map” the distance between 2 genes by comparing how often the cross over.
Low cross over value = not often recombined = close together
SEX LINKED GENES
Carried on the X-chromosome.
As opposed to autosomal.
Males only have 1 allele – more susceptible to recessive diseases, e.g. colour blindness. DO NOT SAY WHAT YOU SEE.
X is normal, Xcis colour blind. Y carries no information.
So… XX is a normal female
What other possibilities can you have?
Explain why it is less common for women to be colour blind.
XcX = normal female XcXc = colour blind female
XY = normal male XcY = colour blind male
In cats one aspect of coat colour is controlled by a sex-linked gene with alleles that are codominant.
These 2 females (XoXo) and (XbXb) are crossed with a male (XbY)…
Draw Punnett squares to find the offspring of each cross.
What is the XbXo offspring called?
How do we get male Tortoiseshell?
Affected (but not dead – yet)
Pleiotropy: One gene ( one protein) controls many phenotypes
40% of cats with white fur and blue eyes are deaf.
Marfan syndrome: one gene is responsible for thinness, joint hyper mobility, limb elongation, lens dislocation, and increased susceptibility to heart disease.
The p53 gene directs damaged cells to stop reproducing, thereby resulting in cell death… helps avert cancer. BUT it also suppresses the division of stem cells, preventing replacement of deteriorating tissues during aging.
Polygeny: Many genes control one phenotype (e.g. Human skin colour, cat coat colour)
Epistasis:Two genes interact, mask, or modify. There are a number of types…
Lab manual page 133
One gene alters the outcome of another
Complementary genes: Both need to be present for either to work.
In a flower, 2 genes control the production of a purple pigment. The intermediate product has no colour.
Can we cross 2 white plants to get purple offspring?
What will the genotype ratio of the dihybrid cross be?
Think 9:3:3:1, but group the last 3 sets.
Supplementary genes:the second gene adds more to the first.
Coat color in Labrador retrievers: two genes (B and E)
A black dogis B_E_
Lab manual page 135/6
This is where 2 genes interact to make a product different to that which either could make independently.
The most common example is comb types in chickens.
See more chickens…
This is where gene expression is affected without changing the underlying DNA code.
e.g. a zygote (toti-potent) becomes more and more specialised by turning different genes on or off.
e.g. Genomic Imprinting.
Some genes can be activated or silenced depending on which parent they came from. The ‘switch’ used to silence or activate a gene is often methylation of cytosines – this inhibits that region of the DNA.
This is where gene expression is affected by the environment.
Some organisms are affected by temperature, altitude or the presence of members of the opposite sex or competitors/predators.
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