6.1.2 Patterns of inheritance

Question 1

Types of variation

Answer 1

• continuous= phenotype in population varies within a range i.e height (polygenic and influenced by environment)
• discontinuous= when there are 2 or more distinct categories the phenotype can fall into (monogenic and not influenced by environment)

Question 2

Environmental factors on phenotype variation

Answer 2

-able to influence gene expression through controlling characteristics i.e chlorosis + animal body mass
-caused by differences in environment

Question 3

Chlorosis

Answer 3

-when plants don’t produce enough chlorophyll + leaves turn yellow/pale
-most plants do show normal genes coding for chlorophyll production but environmental factors affect their phenotype and stop production of proteins i.e
- lack of light(plant stops production to conserve resources)
- mineral deficiencies i.e lack of iron + magnesium means a plant cannot produce chlorophyll
- viral infections(affect metabolism of cells)

Question 4

Animal body mass

Answer 4

-determined by a combination of environmental + genetic factors
-amount + quality of food, quantity of exercise, presence of disease= environmental
-genetic mutations could change the pattern of fat deposition in the body + cause obesity

Question 5

Genetic variation affecting phenotype variation

Answer 5

-created through the individual mix of alleles inherited from both parents–} combination determined by meiosis(crossing over + independent assortment), random fusion of gametes + any mutations in DNA replication
-i.e human blood group

Question 6

Genotypes and phenotypes

Answer 6

• genotype= combination of allele an organism inherits for a characteristic
  -for most genes there are 2 alleles inherited, one from each parent
• phenotype= observable characteristics of an organism
  -any changes the environment makes to an organism’s phenotype are not inherited, they are modifications not mutations(changes to DNA)

Question 7

Definition of gene and allele

Answer 7

-a gene is a sequence of DNA that codes for a protein(resulting in a characteristic)
-alleles are different versions of the same gene –} order of bases in each is slightly different

Question 8

Definition of recessive and dominant alleles

Answer 8

-dominant= the version of the gene that will always be expressed if present in an organism
-recessive= will only be expressed if 2 copies of this allele are present in an organism

Question 9

Definition of homozygous and heterozygous

Answer 9

-homozygous= 2 identical alleles for a characteristic –} could be dominant(2 alleles for dominant phenotype) or recessive(2 alleles for recessive phenotype)
-heterozygous= 2 different alleles for a characteristic
–} dominant allele will be expressed

Question 10

What are genetic cross diagrams?

Answer 10

-show the possible genotypes of an offspring so can be used to predict the genotypes + phenotypes of offspring produced when two parents are bred

Question 11

What is monogenic inheritance?

Answer 11

-inheritance of a characteristic controlled by a single gene
-show the likelihood of different alleles of that gene being inherited by offspring

Question 12

Steps to performing a a genetic cross

Answer 12

-state the phenotype of both parents i.e Green
-state the genotype of both parents i.e GG
-state the gametes of each parent(contain only one allele for each gene)
-use a Punnett square to find potential alleles for offspring
-state the proportion of each genotype + state corresponding phenotype

Question 13

3 rules for monogenic inheritance

Answer 13

-true bred cross (homozygous) will always produce heterozygous offspring
-F1 cross is a cross of offspring from a true bred
-F1 crosses for monogenic always produce a 3:1 phenotypic ratio

Question 14

What is codominance?

Answer 14

-when 2 different alleles occur for a gene and both are equally dominant
-as a result, both alleles of the gene are expressed in the phenotype of the organism if present

Question 15

Snapdragon example of codominance

Answer 15

-snapdragons’ petal colour is controlled by the C gene and its allele can either be red or white:
- Cr Cr (red flowers, homozygous)
- Cw Cw (white flowers, homozygous)
- Cr Cw (pink flowers, heterozygous)

Question 16

What is the expected F1 ratio for codominance?

Answer 16

1:2:1
homozygous: heterozygous: homozygous

Question 17

What are multiple allele crosses?

Answer 17

-some genes have more than 2 versions or multiple alleles–} however as an organism only carries 2 versions of the gene, only 2 alleles will be present in the individual

Question 18

How are human blood groups determined?

Answer 18

-determined by 3 alleles(a, o, b) on gene I
-the gene encodes a specific antigen that will be present on the surface of the RBC
-Ia and Ib are codominant and when both are present, they will both be expressed in the phenotype

Question 19

What is the expected phenotypic ratio for the different blood groups?

Answer 19

-because any offspring could have any one of the 4 blood groups, the ratio would be 1:1:1:1
(A:B:O:AB)

Question 20

What is a dihybrid cross?

Answer 20

-the inheritance of 2 different characteristics, caused by 2 genes that may be located on different pairs of homologous chromosomes
-each of the 2 genes can have two or more alleles
-the crosses can be used to show the likelihood of offspring inheriting certain combinations of the 2 characteristics

Question 21

How many alleles in a genotype and in a gamete?

Answer 21

-4 alleles in a genotype(2 for each characteristic)
-2 alleles per gamete

Question 22

What is the rule for a true bred cross in dihybrid crosses?

Answer 22

-all the offspring produced in the F1 generation will have a heterozygous genotype i.e YyRr

Question 23

What is the expected phenotypic ratio for an F2 dihybrid generation?

Answer 23

9:3:3:1
both dominant: dominant first recessive second: recessive first, dominant second: both recessive

Question 24

What factors can affect the phenotypic ratio?

Answer 24

-crossing over
-random fertilisation of gametes

Question 25

What is sex linkage?

Answer 25

-when the gene coding for a certain characteristic is located on a sex chromosome
-the Y chromosome is smaller than the X chromosome and carries less genes–} males only have 1 X chromosome so often only have 1 allele for sex-linked genes
-therefore they are more likely to express that characteristic in their phenotype even if it is recessive
-if there us a faulty gene on the X chromosome of females, they are more likely to become carriers for the disease + only display it if the faulty gene is on both x chromosomes

Question 26

Haemophilia

Answer 26

-recessive blood disorder carried on the X chromosome
-if a male inherits the recessive allele that codes for haemophilia, they develop the condition
-females who are heterozygous for the haemophilia gene are carriers
-the alleles for the condition are shown alongside the chromosome they are found on i.e Xh

Question 27

What is autosomal linkage?

Answer 27

-occurs on any chromosome that is not a sex chromosome
-genes that are linked when they are on the same chromosome–} they will stay together during independent assortment in meiosis 1 and their alleles will be passed on to the offspring together unless crossing over changes the combination of genes on the chromosome
-linked genes will be inherited together

Question 28

How does crossing over affect autosomal linkage?

Answer 28

-the closer together two genes are on the chromosome, the less likely that crossing over will split them up and vice versa
-offspring affected by crossing over are known as recombinant offspring
-recombinant frequency: no. of recombinant offspring/total no. of offspring

Question 29

What is the expected phenotypic ratio for F1 offspring?

Answer 29

1:1:1:1

Question 30

What is epistasis?

Answer 30

-when the allele of one gene masks the expression of the alleles of another gene
-many genes control the same characteristic + they interact to form the phenotype

Question 31

Examples of epistasis

Answer 31

-widow’s peak is controlled by 1 gene and baldness is controlled by another–} the bald gene would mask the widow’s peak gene
-flower pigment is controlled by 2 genes: yellow pigment + a gene that codes for an enzyme that turns the pigment orange–} yellow pigment is epistatic to colour changing gene(doesn’t matter if you have gene 2 when you do not have the yellow pigment gene)

Question 32

What is the expected phenotypic ratio for recessive epistatic gene in the F2 generation?

Answer 32

-crossing a homozygous recessive parent with homozygous dominant parent results in 9:3:4
-dominant epistatic: recessive other: recessive epistatic

Question 33

What is the expected phenotypic ratio for dominant epistatic gene in the F2 generation?

Answer 33

-crossing a homozygous recessive parent with a homozygous dominant parent would produce a 12:3:1 phenotypic ratio

Question 34

How many alleles in each genotype and gamete for epistasis?

Answer 34

-4 alleles per genotype and 2 alleles per gamete