7A - Genetics

Question 1

Whats genotype and phenotype

Answer 1

GENOTYPE - genetic constitution of an organism, the alleles it has.

PHENOTYPE - the expression of the genetic constitution (genotype) and its interaction with the environment, observable characteristics.

Question 2

What are genes and alleles , inc recessive and dominant

Answer 2

GENE - length of DNA, a sequence of DNA nucleotide bases, 2+ forms and usually code for a polypeptide which results in a characteristic

ALLELE: diff forms of a gene ;usually two alleles for each gene one from each parent
- order of bases slightly differ so code for diff versions of same gene

DOMINANT - allele whose characteristic is expressed in phenotype when only one copy present, capital letter.

RECESSIVE - allele whose characteristic is expressed in the phenotype if 2 copies are present, lowercase letter.

Question 3

Whats a locus and codominant

Answer 3

LOCUS - fixed position a gene occupies on a chromosome, all alleles are found at the same locus on homologous

CODOMINANT - alleles are both expressed in the phenotype, neither is recessive.

Question 4

Whats homo/heterozygous and carrier

Answer 4

HOMOZYGOUS - 2 copies of the same allele AA/aa

HETEROZYGOUS - 2 different copies of alleles Aa

CARRIER - a person carrying an allele which is not expressed in the phenotype but can be expressed in offspring.

Question 5

Genes vs alleles

Answer 5

• A gene is the length of DNA, a sequence of nucleotides that
• normally code for a particular polypeptide.
• ## Polypeptide can be an enzyme that is needed for the production of a characteristic.
• Exist in 2 or more different forms (alleles).
• Alleles occupy the same locus. Only one allele of a gene can occur at the locus of any chromosome.

..
- In diploid organisms, chromosomes occur in homologous chromosomes
- which have 2 loci each carrying one allele of a gene.
- Usually when heterozygous, only one allele is expressed is the phenotype.

Question 6

Hetero vs homozygous

Answer 6

• Heterozygous: alleles on each chromosome are different
• Allele that is expressed when heterozygous is the dominant,
• allele that is not expressed when heterozygous is recessive.

..
- Homozygous: alleles on each chromosome are the same
- Homozygous dominant is when both alleles are dominant so dominant is expressed.
- Homozygous recessive is when both alleles are recessive so recessive is expressed.
- Recessive alleles need to be homozygous recessive

Question 7

ORDER of GENETIC CROSS DIAGRAMS

Answer 7

Alleles
Parent phenotypes
Parent genotypes
Gametes
Punnett square
Offspring genotypes
Offspring phenotypes

Question 8

MONOHYBRID INHERITANCE

Answer 8

diagram1

Monohybrid inheritance is the inheritance of characteristic controlled by a single gene.
> They show the likelihood of the different alleles of the gene being
> inherited by the offspring of specific parents Single letter, capital or lower

..
Eg. GREGOR MENDEL AND PEA POD COLOUR
- Gregor Mendel studied colour of pods of pea plants.
- 2 basic colours of pea plants: green and yellow.
- If pea plants are bred repeatedly with each other and give rise to green pods - pure breeding for green pods.
> These are homozygous
-
- If pure breeding green plants are bred with pure breeding yellow plants,
- all offspring (generation 1, first filial F1) produce green pods.

Green are dominant and yellow recessive

Question 9

DIHYBRID INHERITANCE

Answer 9

diagram 2

• Dihybrid inheritance is the inheritance of 2 characteristics from two different genes on diff chromosomes at same time
• diff combos of characteristic
• Shows how 2 characteristics are inherited at the same time.

..
F1 = only one genotype/phenotype produced
F2 = when two of the F1s are bred together
•
•
- The F1 generation produces 4 types of gametes because the gene for one characteristic
- and the gene for the othe characteristic are on separate chromosomes
-
- So when chromosomes arranged at random on equator during meiosis,
- either one of the 2 alleles for one gene can combines with either of the 2 alleles from the other gene.
-
- Fertilisation is also random, so any of the 4 types of gamete can combine with any of the 4 types
- of gamete from the other plant - further increasing genetic variation.

Question 10

Laws

Answer 10

LAW OF SEGREGATION:
- diploid organisms, characteristics are determined by alleles that occur in pairs.
- Only one of each pair of alleles can be present in a single gamete.
- Rr pair, R in one, r in the other, cant have both in one

..
LAW OF INDEPENDENT ASSORTMENT:
- each member of a pair of alleles may combine randomly with either of another pair.
- RrGg, any R can combine w any of the G’s, it is completely random which one pairs
- The allele a gamete receives for one gene does not influence the allele received for another gene.
-
- When cells divide during meiosis, homologous chromosomes
- are randomly distributed to daughter cells; diff chromosomes
- segregate independently of each other; results in gametes w unique combos of chromosomes.

Question 11

Codominance

Answer 11

diagram 3

BOTH ALLELES ARE EXPRESSED IN THE PHENOTYPE
- Both equally dominant, no recessive
C^ allele
- an allele codes for a protein (or enzyme) which can be functional or structural
- the protein or enzyme produced produces the characteristic

.. EG. Snapdragons
R is red, so proteins produced produces enzyme which makes the red colour.
r is white; faulty enzyme cant produce red (no colour) meaning white
-
- As both red and white are equally dominant, there is codominance and a
- pink colour of snapdragons is produced when both alleles are present

Question 12

MULTIPLE ALLELES

Answer 12

diagram 4

• Where there are more than two alleles, of which only two may be
• present at the loci of an individual’s homologous chromosomes

..
- Although there are three alleles, only two can be present in an individual at any one time,
- as there are only two homologous chromosomes and so only two gene loci.

> Gene^allele
There may be codominance between 2 of alleles
- and one may be recessive (blood groups).

Question 13

Multiple alleles example: blood groups

Answer 13

BLOOD GROUPS
- There are three alleles associated with the gene I (immunoglobulin gene),
- which lead to the presence of different antigens on the cell -surface membrane of red blood cells:

..
- Allele I^A- which leads to the production of antigen A
- Allele 1^B - which leads to the production of antigen B
- Allele 1^O, which does not lead to production of either antigen

The alleles IA and IB are codominant, whereas the allele lO is recessive to both.

Question 14

BLOOD TRANSFUSIONS

Answer 14

• Blood group O has anti-A and anti-B antibodies, so people with type O blood
• can only receive type O blood as their body would reject A and B antigens and cause an immune response.
• Blood group O are universal donors as their red blood cells have no antigens
• on their surface so cannot be rejected.
• Blood group AB are universal recipients as they can receive A and B antigens
• as well as O blood as that has no antigens

Question 15

SEX-LINKAGE

Answer 15

Chromosome ^allele
- Humans have 23 pairs of chromosomes. 22 of these pairs have
- homologous partners that are identical in appearance, whether in a male or a female.
-
- The remaining pair are the sex chromosomes. Female have XX;
Male have XY
- X chromosomes have 153 million base pairs, Y have 50 million base pairs.
-
- The X chromosome is much longer than Y- differs in shape and size
- means for most of length of X chromosome, theres no
- equivalent homologous portion of Y
chromosome

..
-in females: all gametes are same in that they contain a single X chromosome
- in males: produce two diff types of gamete - half have an X chromosome and half have a Y chromosome
-
- Those characteristics that are controlled by recessive alleles on non-homologous portion
- of X chromosome will appear more frequently in the male.
-
>because there is no homologous portion on Y chromosome that might have dominant allele,
> when in the presence of which recessive allele does not express itself.

Question 16

Sex inheritance and haemophilia

Answer 16

Sex inheritance is always 50/50.
- An X-linked genetic disorder is a disorder caused by a defective gene on X chromosome.

..
HAEMOPHILIA diagrams 5
• Blood clots more slowly, potentially lethal if not treated.
> This has resulted in some selective removal of gene from population.
> making its occurrence relatively rare (about one person in 20 000 in Europe).
>
> Although haemophillac females are known, the condition is almost entirely confined to males.
> One of a number of causes of haemophilia is a recessive allele w an altered sequence
> of DNA nucleotide bases on X chromosome that so codes for faulty protein whicqh does not function.

..
> Males always inherit it from their mothers, as Y chromosome from father does not contain allele.
> Females can inherit gene from either parent, they will become carriers if faulty gene is only inherited from one.

Question 17

AUTOSOMAL LINKAGE

Answer 17

WHEN GENES OCCUR ON SAME CHROMOSOME THEY ARE LINKED
> Single letter, capital or lower.

• In humans, just 23 pairs of chromosomes carry the genes that
• ## determine many thousands of different characteristics.
• It follows that each chromosome must possess many diff genes.
• Any two genes that occur on same chromosome are said to be linked.
• All genes on a single chromosome form a linkage group.
• Genes carried on the sex chromosomes are said to be sex linked.

..
- The remaining 22 chromosomes, other than the sex chromosomes, are called autosomes.
- name given to situation where two+ genes are carried on same autosome is called autosomal linkage.
-
- Assuming there is no crossing over, all linked genes remain together during meiosis
- and so pass into gametes, and hence the offspring, together.
-
- They do not segregate in accordance with Mendel’s Law of Independent Assortment.

Question 18

Linked vs not linked (autosomal linkage)

Answer 18

diagram 6

• When linked, there are only 2 possible gametes, provided there is no
• crossing over, because the alleles are on the same chromosome

..
- When not linked, there are 4 possible gametes because gene for one characteristic
- and the gene for the other characteristic are on separate chromosomes.
-
- So when chromosomes arranged at random on equator during meiosis, either one of
- the 2 alleles for one gene can combine with either of 2 alleles from the other gene.
»This is dihybrid inheritance.

Question 19

How to interpret recombinants using % recombination

Answer 19

• When two genes are on same chromosome, they are linked and tend to be inherited together.
• BUT sometimes crossing over occurs during meiosis, producing recombinant gametes
• recombinant offspring
  > offspring with new allele combinations that the parents did not have.
• AB / ab are Non-recombinants
• AB and ab (same combinations as parents)
• Ab and aB are recombinants
• (new combinations → produced by crossing over)

..
gives you offspring numbers and expects you to calculate:

Recombination freq=
number of recombinants/tot offspring
x 100

..
A higher % recombination means:
• more crossing over occurred
• the genes are further apart on the chromosome

A lower % recombination means:
• less crossing over occurred
• the genes are closer together

Question 20

Epistasis

Answer 20

• Epistasis arises when allele of one gene affects or masks
• ## expression of another in the phenotype.
• Gene A codes for one characteristic. has dominant or recessive
• ## Gene B codes for another characteristic, also has dominant or recessive.
• If gene A controls the colour of something, and gene B controls the production of pigment,
• then if gene B is bb (recessive) then no pigment is produced
  -which means regardless of the genotype of gene A, there will be no colour.

Question 21

Epistasis example

Answer 21

• Gene 1: C / c — “Colour production gene”
  •C = pigment can be produced
  •c = pigment cannot be produced (faulty enzyme in the pigment pathway)

Genotypes
•C_ = pigment made
•cc = no pigment at all → albino

• This gene is epistatic because if the mouse is cc, it doesn’t matter what
• the second gene is — no pigment is made, so no colour can appear.

⸻

Gene 2: A / a — “Colour distribution gene”
(decides what colour pigment will be, only if pigment exists (i.e., only if C is present).
•A_ = agouti (banded) fur
•aa = black fur

But this only shows if the mouse is C_.

⸻

If a mouse is cc
•It produces no pigment, so the coat is albino, regardless of whether it is AA, Aa, or aa.
•So cc masks the effect of A/a.

→ C/c gene is epistatic over A/a gene.
→ A/a gene is hypostatic (its effect is hidden if cc is present).

Question 22

WHY ARE GENETIC CROSSES RARELY THE SAME AS PREDICTED RESULTS?

Answer 22

• It is chance that determines which gametes fuse due to statistical error
  > Each time gametes fuse is an independent event - still a certain chance but its random.

> Small population/sample
Selection advantage or disadvantage/lethal alleles

Larger sample= more likely cross results are close to actual results

> Larger sample = more likely cross results are close to actual results

Question 23

CHI-SQUARED TEST

Answer 23

diagram7

> Statistical test to find out whether diff between observed v
expected data is due to chance, used to test the null hypothesis.when to use chi-squared test
- data are in categories (i.e. discrete variation)
- The sample size must be relatively large, over 20
- The data indicate absolute numbers (frequencies not %)
- It is used to compare experimental results with theoretical ones
-
- Due to random nature of gamete fusion, these are rarely 100% accurate predictions

..
How to apply the chi-squared test:
1. Define null hypothesis - no significant diff between observed/expected data/frequencies
2. Calculation of chi-squared value
3. Determine number of degrees of freedom (number of categories - 1)
4. Determine critical value at p = 0.05 (5% probability) from a table
5. Interpreting the results

Question 24

INTERPRETING CHI-SQUARED VALUES

Answer 24

1 Calculated value of Chi-squared is larger than the critical value at p = 0.05

2. Hence there is less than 5% probability that the differences
   - between the observed and expected data are due to chance i.e. difference is significant

3 Reject the null hypothesis

4. Calculated value of Chi-squared is smaller than the critical value at p = 0.05
5. Hence there is more than 5% probability that diffs between
   - observed and expected data are due to change i.e. not significant
6. Accept the null hypothesis