6 - ICH - Genetics & Inheritance Flashcards
Define Genotype
Genotype = The genetic makeup of an organism and describes the organism in terms of the alleles it contains
Define Phenotype
Phenotype = Observable characteristics of an organism. It is the result of the interaction between the genotype and the environment which can modify the organism’s appearance
Define Gene
A DNA sequence that codes or a polypeptide
Define Allele
Different forms of the same gene
Define Homozygous and Heterozygous
Homozygous = Body cells in an organism have 2 identical alleles for a gene and all gametes carry the same allele
Heterozygous = Body cells in an organism have 2 different alleles for a gene and gametes will carry one copy of one of the alleles
Define
Dominant
Recessive
Co dominant/ incomplete dominance
Dominant = Allele if present will always be expressed in the phenotype
Recessive = Allele is only expressed in phenotype in the homozygouse state
Co dominant/ incomplete dominance = Both alleles are equally dominant and are both expressed in the phenotype
Define Locus
Locus = The specific position of a gene on a DNA molecules/ chromosome
What are genetic diagrams and their purpose?
Genetic diagrams show the possible genotypes and phenotypes of the offspring that would be produced if two parents are crossed
What is monohybrid inheritance? + Example
Inheritance of a characteristic controlled by a single gene
- E.g. Tall/ short pea plants
What is another name given to monohybrid inheritance?
Monogenic inheritance
If 2 homozygous parents are crossed in a monogenic cross. What is expected of the genotypes of the F1 (first generation offspring)
They’ll all be heterozygous
What is a phenotypic ratio?
Phenotypic ratio = The ratio of different phenotypes in the offspring
Name 3 different types of genetic cross diagrams you need to know + example of each
Monohybrid inheritance - Inheritance of 1 gene
- Tall/ short pea plants
Dihybrid inheritance - Inheritance of 2 genes
- Shape and colour of pea plants
Multiple allele inheritance
- Blood group
Why is blood group expressed as IA, IB and IO
What does the I stand for?
Immunoglobin
- How many pairs of chromosomes do humans have?
- What are autosomes and how many pairs do humans have?
- Humans have 23 pairs of chromosomes
- Autosomes = pairs of chromosomes that are identical in appearance (humans have 22 pairs)
XY chromosomes for a male and female?
Female = XX
Male = XY
Can any gene on a sex chromosomes be sex-linked?
Yes
Can alleles be carried on the X and Y chromosome?
What is particular about the alleles expressed and the X chromosome?
Y = Carries no alleles
X = Carries a allele ∴ in males often when the allele on X is recessive it will still be expressed since there’s no allele on the Y chromosome
Draw a genetic cross diagram to illustrate the inheritance of sex
Monohybrid cross diagram
What a gene being called sex-linked mean?
Sex-linked = The allele is carried on either the X or Y chromosome but in practise very few is carried on the Y chromosome (we assume none for genetic crosses)
If a genetic disease is carried on the X chromosome what sex has a greater predisposition for the disease? Why?
Males, because the Y chromosome doesn’t really carry any alleles (assume none in questions) ∴ the allele carried on the X chromosome regardless of whether it is recessive or dominant will be expressed
Name 2 diseases that are sex linked
- Is the faulty allele dominnt or recessive
- What sex has a greater predisposition for the disease?
- Haemophilia and red/ green colour blindness
- Alleles responsible for these diseases are recessive
What is haemophilia
Condition in which the blood does not clot, leading to slow and persistent bleeding, especially into joints
What is red green colour blindness?
Difficulty distinguishing between red and green.
Gene controlling red green colour vision is located on the X chromosome
Question - Sex linkage:
A man, which is red-green colour blind marries a woman who is homozygous for the allele for normal vision. Show by means of a genetic diagram, the possible genotypes and phenotypes of their children
Question - Sex linkage:
Cross a female carrier of haemophilia with a normal male
ABO blood group is an exmaple of what? Why?
Multiple allele inheritance and codominance.
IA and IB are codominant
Both IA and IB are dominant to IO
Name 2 situations that illustrate monogenic inheritance of codominant alleles
Sickle cell anaemia
Pink flowers
Possible genotypes for blood group:
- A
- B
- O
Blood group A = IAIA and IAIO
Blood group B = IBIB and IBIO
Blood group O = IOIO
There are 3 alleles for blood groups A,B and O. At any given time can one carry all 3?
No
Blood groups are composed of 3 sections. What are they?
-
ABO
- A and B are co-dominant
- A and B are both dominant to O
-
MN co-domidance
- M = MM
- N = NN
- MN = MN
-
Rhesus factor
- Positive or negative
Question:
A sex-linked, dominant allele, barred (feather pattern) can be used to sex newly hatched chicks, the offspring of a barred hen mated with a non-barred rooster.
Note! In birds, the female is the heterogametic sex (XY) and the male the homogametic sex (XX).
- Construct a genetic cross diagram of this cross.
- What is the genotype of the female chicks?
Question:
A mother with blood group A, whose father was blood group O, marries a man with blood group AB.
What is the probability that their first child will be a girl with blood group B?
What happens when there’s more than 3 alleles for a particular gene?
The allelesa re arranged in a hierarchy with each allele being dominant to the ones below it and recessive to thoese above it.
They will give you the order of dominance
when does dihybrid inheritance occur?
When 2 characteristics are controlled by 2 genes on seperate chromosomes
For dihybrid crosses what is the most common phenotypic ratio along with the parental genotype?
9 : 3 : 3 : 1
Both parents are heterozygous e.g. RrYy x RrYy
What do you do when after drawing the genetic diagrams to predict the phenotypes of F1 individuals, the expected and observed phenotypes are significantly different what do you do?
Chi-Squared
Define:
Linkage
Autosomal linkage
Sex linkage
Linkage = When 2 or more genes are located on the same chromosome
Autosomal linkage = Linked genes which are non-sex chromosomes
Sex linkage = Linked genes are on sex chromosomes
Define Epistasis
Epistasis (interactions between gene loci) = Interaction of non-linked genes where one masks the expression of the other
What can epistasis result in?
- Genes working against each other (antagonistically) ∴ results in masking
- Genes working together in a complementary fashion
Relationship between the epistatic gene and the hypostatic gene
- Epistatic gene at one locus alters or inhibits the expression of a second locus, the hypostatic gene.
- Epistatic gene produced a protein which influences or controls the expression of the hypostatic gene
How does epistasis allow genes to work antagonistically?
- Homozygous presence of a recessive allele may prevent the expression of another allele at a second locus
- Alleles at the first locus = epistatic
- Alleles at the second locus = hypostatic
Epistasis is not inherited, it is an interaction between 2 gene loci. It reduces phenotypic variation
What does epistasis reduce?
Phenotypic variation
Explain the concept of recessive epistasis + example
Occurs when recessive alleles at one locus stop the expression of alleles at a second locus
- E.g. Flower colour in Salvia
- 2 genes on different chromosomes control flower colour A/a and B/b (2 loci)
- B = purple flowers, b = pink flowers, but if another gene is recessive aa, then no colour can be made (white flowers are produced regardless of what is on B/b gene)
Explain the concept of dominant epistasis + example
Occurs when a dominant allele at one locus stops the expression of the alleles at a second locus
-
E.g. inheritance of feather colour in some breeds of chickens
- I/i and C/c are 2 gene loci that control feather colour
- C = coloured feathers, c = white feathers
- I = white feathers even if they have the dominant allele C
- cc = alsp produce white feathers regardless of what is on I/i gene
So if you cross 2 chickens IICC x iiCC
- All of F1 = IiCc ∴ all white feathers
If F1 interbreeds IiCc x IiCc
- F2: white feathered : coloured feathered in ratio 13:3
How does epistasis allow genes to work complementary with each other? + example
Works like a general metabolic pathway:
- 2 genes affect the colour of sweet pea flowers, Allele A and Allele B
- Colourless precursor moelcule is the base of what you start with
- If allele A is present, it produces enzyme A ∴ reaction can continue to second step etc..
- Purple flowers = A and B is present
- Red flowers = A must be present but B is absent
- If A is absent then flowers are white
Equation for Chi-squared = ?
When do you reject Ho and accept Ho?
To test for whether the difference between observed and expected values are significant or by chance
- Probability level = 5% = 0.05 (unless stated otherwise)
- Degree of freedom = n - 1
Xcritical > Chi calculated - Accept Ho
Xcritical ≤ Chi calculated - Reject Ho
