Inherited Change

Question 1

What is a gene?

Answer 1

A sequence of based on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic.

Question 2

What is an allele?

Answer 2

A different version of a gene. There can be many different alleles of a single gene.

Most plants and animals, including humans, only carry two alleles for each gene, one from each parent.

The order of bases in each allele is slightly different - code for different versions of the same characteristic.

B = allele. b = allele.

Question 3

Genotype?

Answer 3

The genetic constitution of an organism - the alleles an organism has.

Bb is a genotype. bb is a genotype.

The whole gene, not just looking at one allele.

Question 4

Phenotype?

Answer 4

The expression of the genetic constitution and its interaction with the environment - an organisms characteristics.

Brown eyes is a phenotype.

Question 5

Codominant?

Answer 5

Alleles that are both expressed in the phenotype.

Neither gene is dominant or recessive.

Question 6

Locus?

Answer 6

The fixed positive of a gene on a chromosome.

Allies of a gene are found at the same locus on each chromosome in a pair.

Question 7

Homozygote and hetrozygote?

Answer 7

Homozygote - an organism that carries two copies of the same allele.

E.g. BB or bb.

Heterozygote - an organism that carries two different allele.

E.g. Bb.

Question 8

Carrier?

Answer 8

A person carrying an allele which is not expressed in the phenotype but that can be passed onto the offspring.

Question 9

Diploid and haploid?

Answer 9

Diploid - have two sets of chromosomes, 23 pairs.

Haploid - 23 chromosomes.

Question 10

What is a gamete?

Answer 10

A sex cell.

When gametes from two parents fuse together, the alleles they contain form the genotype of the offspring.

At each locus, the genotype can be homos I guess or heterozygous.

Question 11

Genetic diagrams?

Answer 11

Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parents are crossed.

Question 12

EXAM QUESTIONS TO DO?

Answer 12

Practising exam questions for this topic is the most efficient way to learn.

Exam questions on google classroom 15/3 McAllister PowerPoint.

Include: 
Monohybrid genetic crosses,
Co-dominance genetic crosses, 
Multiple alleles genetic crosses (blood groups),
Sex-linkage genetic crosses.
Pedigree charts,
Dihybrid inheritance genetic crosses,
Autosomal linkage genetic crosses,
Epistasis genetic crosses,
Chi-squared.

Question 13

Monohybrid crosses and dihybrid crosses?

Answer 13

A monohybrid cross shows the inheritance of a characteristic controlled by a single gene.
E.g. wing length. Short wings - N and long wings - n.
When places into a punnet square, they should have only 2 letters in each box.
(NN x nn)

A dihybrid cross shows the inheritance of a certain combination of characteristics. You can use a dihybrid cross to look at how to different genes are inherited at the same time.
E.g. wing length and colour. Short wings - N and long wings - n. Pink wings - P and blue wings - p.
When put into a punnet square, there should be four letters in each box. 2 letters above and to the side of each box.
(NNPP x nnpp)

Question 14

Punnet square: female and male genotypes should be where?

Answer 14

The females phenotypes should always go at the top of the punnet square. The males phenotypes should always go to the side of the punnet square.

Question 15

Codominant allele cross diagrams?

Answer 15

Both alleles are expressed in the phenotype. This means that neither one is recessive or dominant. One example in humans is the allele for sickle-cell anaemia.

This looks like a big, capital letter and then a small capital letter to the right of the big letter.

H^H

People who are homozygous for normal haemoglobin don’t have sickle-cell anaemia (H^N, H^N).

People who are homozygous for sickle haemoglobin have sickle-cell anaemia. This means all their blood cells are sickle shaped. (H^S, H^S).

People who are heterozygous have an in between phenotype, called the sickle-cell trait. (H^N, H^S).
They have some normal haemoglobin and some Seco haemoglobin. The two alleles are codominant because they’re both expressed in the phenotype.

Question 16

Multiple allele genetic crosses?

Answer 16

In humans we have a ABO blood-group system.

There are three alleles for blood type:
I^O - blood group O.
I^A - blood group A.
I^B - blood group B.

Allele IO is recessive.

Alleles IA and IB are codominant - people with the geneotype IAIB will have the blood group AB.

To make things less complicated, split the genotype into gametes first.
E.g. genotype: IAIO.
Gametes: IA and IO.
Then put these gametes into the genetic cross.

Question 17

Phenotype ratio for all types of crosses when combining a homozygous dominant and a homozygous recessive?

Answer 17

Monohybrid: Crossing a homozygotes dominant and a homozygotes recessive = all heterozygous offspring.
Ratio is 3:1.

Dihybrid: Crossing a homozygotes dominant and a homozygous recessive = all heterozygous offspring.
Ratio is 9:3:3:1.

Codominant: Crossing a homeless I guess for one allele with a homozygous for the other allele = all heterozygous offspring.
Ratio is 1:2:1.

This is really good to know when you’re stuck. However, it is much better to work it out yourself because that way you are able to know what the phenotypes for the ratios are, and not just what the ratio is.

Question 18

Why might the expected phenotypic ratio not occur?

Answer 18

When you finish a genetic across diagram, you can predict the phenotypes of the offspring.

However, the prediction is not always right.

This can be because of sex linkage, autosomal linkage or epistasis.

Question 19

Sex-linked characteristics?

Answer 19

The information for biological sex is carried on two sex chromosomes.

In mammals, females have two X chromosomes (XX) and males have one Y and one X chromosome (XY).

The Y chromosome is smaller than the X and so carries fewer genes. So most genes on sex chromosomes are carried on the X chromosome and they’re called X-linked genes.

Males only have one X chromosome and so they often only have one allele for sex-linked genes. They only have one copy and so they express the characteristics of this allele even if it is recessive. This makes males more likely than females to show recessive phenotypes for genes that are sex-linked.

Genetic disorders caused by faulty alleles on sex chromosomes include colour blindness and haemophilia. The faulty allies for both of these disorders are carried on the X chromosomes and they’re called sex-linked disorders.

SHOWN: XNXn - carrier female.
XnY - colour blind male.

Females would need two copies of the recessive allele to have a sex-linked disorder. While males only need one copy. This means that males are more likely to have the sex-linked disorder.

Question 20

Autosomal genes?

Answer 20

Autosome - chromosome that isn’t a sex chromosome.

Autosomal genes - genes located on the autosomes.

Genes on the same autosome are said to be linked because they’re on the same autosome and so they’ll stay together during independent segregation in meiosis 1 and their alleles will be passed on to the offspring together. If you get one, you get the other.

The only reason this wouldn’t happen is if crossing over split them. The closer together two genes on the chromosome are, the less likely it is for them to be split up.

If two genes are autosomally linked, you won’t get the expected ratio for offspring.

Question 21

Genetic crosses showing autosomal linkage?

Answer 21

Example:

Gene for normal wings - N
Gene for wonky wings - n
Gene for red eyes - R
Gene for white eyes - r

He crosses heterozygous dominant for red eyes and normal wings (NnRr) with homozygous recessive for wonky wings and white eyes (nnrr).

The expected ratio would 1:1:1:1
But instead, they got 8:1:1:8
Normal wings and red eyes were 8, and vestigial wings and white eyes were 8. The other two options were 1’s.

The reason that the ratio is so off (and so many of the offspring were red eyes and normal wings and white eyes and wonky wings) is because the genes are linked.

Some of the other offspring were produced (not many though) due to crossing over but there are so many fewer 1 ratio offspring, that sex-linked genes must occur here.

Linked genes - normal wings and red eyes.
Linked genes - wonky wings and white eyes.

Question 22

Epistatic genes?

Answer 22

Many different genes might interact to control the same characteristic phenotype. This can be because of an allele of one gene masks (blocks) the expression of the alleles of other genes. This is called epistasis.

Example:
In humans, you can get a a gene that codes for widows peak (up and down hairline). However, you might also have a gene for baldness. This gene would mask the gene for widows peak.

Example 2:
In flowers, flower pigment is controlled by two genes. Gene 1 codes for a yellow pigment and gene 2 codes for an enzyme that turns the yellow pigment orange. Without the yellow allele, orange allele won’t be able to work. So if you don’t have gene 1, the flower will be colourless.

Gene 1 is epistatic to gene 2 because it’s masks the expression of gene 2.

Crosses involving epistatic genes don’t result in the expected phenotypic ratios you’d normally expect.

Question 23

Predicting phenotypic ratios for epistatic genes?

Answer 23

A dihybrid cross involving a recessive homozygous parent with a homozygous dominant parent - 9:3:4 - dominant both : dominant epistatic recessive other : recessive epistatic.

A dihybrid cords involving a homozygous dominant epistatic allele with a homozygous dominant parent will produce a 12:3:1 - dominant epistatic : recessive epistatic dominant other : recessive both.

Question 24

What is the chi-squared test?

Answer 24

Statistical test that’s used to see if the results of a study support a theory. If the results are significant.

1. Write a theory - this is used to predict a expected result. E.g. wing length in fruit flies is controlled by a single gene with two alleles (monohybrid inheritance).
2. Write expected results - E.g. if you cross a homozygous dominant parent with a heterozygous recessive parent, you’d expect a 3:1 phenotypic ratio of normal:wonky wings in the F2 generation.
3. Write observed results - the experiment is carried out and the number of F2 offspring is counted.
4. Write a null hypothesis. There is no significant difference….
5. Do the X2 test to decide if you should accept or reject the null hypothesis. If the X2 test shows that observed and expected results are not the same, then we cannot reject the null.

Question 25

Chi-squared formula?

Answer 25

X2 = { (O-E)2 / E

O = observed result. 
E = expected result.
2 = squared sign.
{ = sum off. 

X2 = chi-squared.

Do practice exam questions for this.

Question 26

How to find out if you should accept or reject the null hypothesis in chi-squared test?

Answer 26

If the X2 value is greater than or equal to the critical value, then there is a significant difference.

We reject the null and accept the alternative.

You might be given the critical value or expected to calculate it in the exam.

We use the probability level of 0.05 (5%). This means there is a 5% probability that the results are due to chance.

Finding critical value:
Degrees of freedom = number of phenotypes minus one.
Find the 0.05 and degrees of freedom number on the chart. Go along to find critical value.

Question 27

Anatomical and physiological adaptations?

Answer 27

Anatomical - adaptation to the organisms physical structure/body.

Physiological - adaptation to the process that goes on inside an organisms body.