Section 7 - Genetics, populations, evolution and ecosystems: 17. Inherited change

Question 1

What is an organism’s ‘Genotype’

Answer 1

The genetic constitution of an organism
- Includes all the alleles that an organism has
- Determines the limits, within which an individual may vary
eg. Genes mean max height is 6ft, but actual height is effected by other factors, such as diet

Question 2

What is an organism’s ‘Phenotype’

Answer 2

The observable/biochemical characteristics of an organism
- Result of the interaction between the expression of the genotype and the environment
- ∴ Altered by environmental factors
eg. Maximum potential height may not be reached, due to factors such as a calcium deficiency

Question 3

What is a ‘Gene’

Answer 3

Length of DNA (sequence of nucleotides) that codes for a protein
- Codes for polypeptide that is folded into protein
- Produced proteins determine the phenotype

Question 4

What is a ‘locus’

Answer 4

The position of a gene on a chromosome

Question 5

What is an ‘Allele’

Answer 5

One of the several different forms of a gene
- Only one allele of a gene can occur at the locus of a chromosome
- Alleles can be recessive or dominant
- In diploid organisms with homologous chromosomes (2 copies of each), there are 2 loci for each gene, each with a certain allele

Question 6

What is Monohybrid inheritance

Answer 6

The inheritance of a single gene, with one dominant allele and one recessive allele

Question 7

How are alleles written for monohybrid inheritance

Answer 7

• Upper case letters show the dominant allele
• Lower case letter show the recessive allele
• 2 Alleles written for diploid organisms
• Both are the same letter
• eg. Pea pod colour:
  
  • G = Green
  • g = Yellow

Question 8

What does it mean if an organism is ‘Heterozygous’ for a particular characteristic

Answer 8

The organism has one dominant allele, and one recessive allele for the gene
- Eg. Pea pods (G = Green, g = Yellow)
- Heterozygous: Gg
- ∴ Phenotype = Green, as dominant allele is expressed

Question 9

What does it mean if an organism is ‘Homozygous’ for a particular characteristic

Answer 9

The organism has 2 of the same allele for the gene
- Eg. Pea pods (G = Green, g = Yellow)
- Homozygous Dominant: GG
∴ Phenotype = Green
- Heterozygous Recessive: gg
∴ Phenotype = Yellow

Question 10

How are genetic crosses shown on a Punnet square

Answer 10

• Each possible gamete from one parent is placed at the top of each column
• Each possible gamete from the other parent is placed at the end of each row
• ∴ All possible genetic crosses can be seen by combining the gametes of both parents in all possible ways
• Allows genotype and phenotype ratios to be determined (probability that off-spring will have each possible combination)

Question 11

What are the genotype and phenotype ratios of the off-spring from a cross between two heterozygous parents in monohybrid inheritance (eg. Pea colour)

Answer 11

Gg x Gg
(Gametes for each parent: G, g)

• Genotype:
  
  • GG : Gg : gg = 1:2:1
• Phenotype:
  
  • Dominant : Recessive = 3:1

Question 12

How do you determine the genotype of an organism with a dominant phenotype
(Could be Heterozygous or Homozygous dominant…)

Answer 12

Test Cross:
Cross the organism that has the Dominant phenotype (Unknown genotype) with an organism that has a Recessive phenotype (Recessive genotype)
- If organism with dominant phenotype is Homozygous dominant:
- All offspring will be heterozygous, as each will have one dominant allele from the homozygous dominant parent
- ∴ All off-spring will have dominant phenotype
- If organism with dominant phenotype is Heterozygous:
- Half of the off-spring will be heterozygous and half will be homozygous recessive, as half will have inherited a dominant allele from the heterozygous parent and half will have inherited a recessive allele from them
- ∴ Half of the off-spring will have dominant phenotype, and half will have recessive phenotype

Can also be used for dihybrid inheritance, just considering the traits separately as if for monohybrid inheritance

Question 13

What is Dihybrid inheritance

Answer 13

The inheritance of two different genes, located on two different chromosomes

Question 14

How are alleles written for dihybrid inheritance

Answer 14

• Upper case letters show the dominant allele
• Lower case letter show the recessive allele
• 2 alleles written for each gene, for diploid organisms (4 total in dihybrid inheritance)
• Different letters used for different genes
• eg. Pea pod ‘colour’ and ‘shape’:
  
  • G = Green
  • g = Yellow
  • R = Round
  • r = Wrinkled

Question 15

What are the phenotype ratios of the off-spring from a cross between two parents heterozygous for both traits in dihybrid inheritance (eg. Pea colour and shape)

Answer 15

GgRr x GgRr
(Gametes for each parent: GR, Gr, gR, gr)

• Phenotype:
  
  • Dominant & Dominant : Dominant & Recessive : Recessive & Dominant : Recessive & Recessive
    = 9:3:3:1

Question 16

What is Mendel’s ‘Law of independent assortment’

Answer 16

Each member of a pair of alleles can combine randomly with any other when inherited
- First recognised as the observed phenotype ratios of the off-spring from a genetic cross matched that of the expected ratio

Question 17

What is Codominance

Answer 17

Inheritance in which both alleles are expressed in the phenotype of the offspring (Both are equally dominant)

Question 18

How are alleles written when they are codominant

Answer 18

• Gene is written as the base (eg. C = colour)
• Superscript shows the allele
• All alleles are capitals, to show they are equally dominant
• Each allele is a different letter
• eg. C^(R) = Red
• eg. C^(W) = White

Question 19

What is the Phenotype of an organism that is heterozygous for an organism, with 2 codominant alleles

Answer 19

If an organism has 2 different, codominant alleles for the same gene, both traits are expressed.
- eg. Flower colour
- C^(R)C^(R) = Red
- C^(W)C^(W) = White
- Heterozygous: C^(R)C^(W) = Pink (mix of both traits)

Question 20

What are the genotype and phenotype ratios of the off-spring from a cross between two heterozygous parents in codominant inheritance (eg. Flower colour)

Answer 20

C^(R)C^(W) x C^(R)C^(W)
(Gametes for each parent: C^(R), C^(W))

• Genotype:
  
  • C^(R)C^(R) : C^(R)C^(W) : C^(W)C^(W) = 1:2:1
• Phenotype:
  
  • Red : Pink : White = 1:2:1

Question 21

What is inheritance involving ‘multiple alleles’

Answer 21

When a gene with more than two possible alleles is inherited
- However, there are still only 2 loci for each gene in diploid organisms
- eg. Gene for blood type

Question 22

How are alleles written for inheritance involving ‘multiple alleles’
(eg. blood groups)

Answer 22

• Gene is written as the base (eg. I = Immunoglobin gene)
• Superscript shows the allele
• Alleles can be capital or lowercase to show if they are dominant or recessive
• If two dominant are present, mixed phenotype is seen like in codominance (eg. Blood group AB)
• Each allele is a different letter
• eg. I^(A) = Red blood cells have surface antigen A (dominant)
• eg. I^(B) = Red blood cells have surface antigen B (dominant)
• eg. I^(o) = Red blood cells have surface antigen o (recessive)

Question 23

What is ‘Sex-linked’ inheritance

Answer 23

Inheritance of a Gene located on either the X or Y chromosome

Question 24

What is the difference between the sex-chromosomes of Males and Females

Answer 24

Males = XY
Females = XX

Question 25

What are X-linked genetic disorders

Answer 25

Genetic disorders caused by a defective gene on the
X-chromosome
- X-chromosome is much longer than the Y chromosome, so there are many alleles on the X, without a corresponding locus on the Y
- If the disorder is caused by a recessive allele, will be more common in men, as there is only one X-chromosome (any allele present will be expressed)
- eg. Haemophilia

Question 26

What is Haemophilia

Answer 26

X-linked genetic disorder caused by a recessive allele that codes for a faulty protein, so the individual is unable to form proteins required for blood clots
- ∴ Blood clots slowly resulting in persistent internal bleeding
- More common in males
- Caused by recessive allele, and males only have one X-chromosome
- Condition tends to kill females at the onset of menstruation

Question 27

What is a Pedigree chart

Answer 27

Diagram that shows the inheritance of sex-linked characteristics

Question 28

What are Autosomes

Answer 28

Any chromosome that isn’t a sex chromosome

Question 29

What is Autosomal Linkage

Answer 29

When 2 genes are on the same chromosome, they are said to be linked as, if no crossing over occurs, one can’t be inherited without the other
- Autosomal linkage = Both genes on the same non-sex chromosome
- Crossing over is less likely to separate the linked genes if they are close together

Question 30

What is the impact of Autosomal linkage on inheritance

Answer 30

If 2 genes are on the same chromosome, they don’t segregate randomly according to Mendel’s ‘Law of independent assortment’
- eg. If gene A is on one chromosome (homologous pair with alleles A and a) and gene B is on another chromosome (homologous pair with alleles B and b), there are 4 possible gametes that can form:
- AB, Ab, aB, ab
- However, if genes A and B are on the same chromosome (homologous pair with one having the alleles AB and the other having the alleles ab), there are only 2 possible gametes that can form, as the linked genes must be inherited together:
- AB, ab
- This will effect the expressed phenotypes, as certain genotypes are impossible in the offspring
- Genes may be linked in many different combinations, so each will effect the off-spring phenotypes differently
(eg. ‘Ab’ on one chromosome and ‘aB’ on it’s homologous pair, meaning only the gametes Ab and aB are possible)

Question 31

What is Epistasis

Answer 31

When the allele of one gene affects and masks the expression of another gene in the phenotype
- Normal dihybrid cross occurs
- The phenotype ratio is different as the expression of one gene will alter how the other is expressed
- eg. Both code for the same characteristic

Question 32

What are some examples of inheritance involving epistasis

Answer 32

eg. Mice hair:
- Gene A controls the distribution of melanin (black pigment)
- Allele A = Hair has bands (agouti)
- Allele a = Hair is solid black
- Gene B determines if melanin is produced
- Allele B = Melanin is produced
- Allele b = No Melanin is produced
- ∴ The expression of gene A is effected by gene B, as if no melanin is produced, the mouse will be white, regardless of what alleles there are for gene A

eg. Biochemical pathways
- Gene A codes for enzyme A, which produces an intermediate molecule from a starting molecule
- Allele A = Functional enzyme A
- Allele a = Dysfunctional enzyme A
- Gene B codes for enzyme B, which produces a final molecule from the intermediate molecule
- Allele B = Functional enzyme B
- Allele b = Dysfunctional enzyme B
- ∴ The expression of gene B is effected by gene A, as if enzyme A is dysfunctional and no intermediate molecule is produced, the final molecule won’t be made, regardless of what alleles there are for gene B

Question 33

What is the ‘Chi-squared’ test

Answer 33

Statistical test used to determine if any deviation between the observed and expected results of an experiment are significant or not
- Used for genetic crosses (compare observed and expected phenotype ratios)

Question 34

What is the equation for the ‘Chi-squared’ test

Answer 34

x^2 = Σ ((O-E)^2)/E

x^2: Chi-squared value (compared to critical value from table)
O: Observed number for particular result
E: Expected number for particular result
Σ: Sum of value for all results

Question 35

What criteria must be met for a ‘Chi-squared’ test to be valid

Answer 35

• Sample size is relatively large
• Data must be in discrete categories
• Only raw counts are used (not rates, percentages, etc)
• Used to compare experimental and theoretical results

Question 36

What are is the degree of freedom for a ‘Chi-squared’ test

Answer 36

Number of independent pieces of information used to calculate the chi-squared value
(No. of categories - 1)

Question 37

How do you interpret the results of a ‘Chi-squared’ test (critical value)

Answer 37

Using a chi-squared table:
- Row = Degrees of freedom (No. of categories - 1)
- Column = p-Value (Accepted as 0.05)
- This will give a critical value
- If x^2 > Critical value, reject Ho
- If x^2 < Critical value, accept Ho