Module 6: Section 2 - Patterns of Inheritance

Question 1

What is continuous variation?

Answer 1

continuous variation is when the individuals in a population within a range - there are no distinct categories e.g. waist circumference / height

Question 2

What is discontinuous variation?

Answer 2

discontinuous variation is when there are two or more distinct categories, there are no intermediates, e.g. blood group, or violet flower colour - they can be violet or white

Question 3

How can variation be influenced by your genes?

Answer 3

1) different species have different genes
2) individuals of the same species have the same genes, but different versions of them (called alleles)
3) the genes and alleles an organism has make up its genotype
4) sexual reproduction leads to variation in genotypes within a species. Meiosis makes gametes with a unique assortment of alleles through crossing-over and the independent assortment of chromosomes. the random fusion of gametes during fertilisation also increases genetic variation in the offspring
5) the differences in genotype result in variation in phenotype - the characteristics displayed by an organism. variation in phenotype is also referred to as phenotypic variation
6) inherited characteristics that show continuous variation are usually influenced by many genes - these characteristics are said to be polygenic
7) inherited characteristics that show discontinuous variation are usually influenced by only one gene. Characteristics that are only controlled by one gene are said to be monogenic

Question 4

Characteristics that are only controlled by one gene are…?

Answer 4

characteristics that are only controlled by one gene are said to be monogenic

Question 5

Define polygenic characteristics

Answer 5

inherited characteristics that show continuous variation are usually influenced by many genes - these characteristics are said to be polygenic

Question 6

define gene

Answer 6

a sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic

Question 7

define allele

Answer 7

a different version of a gene. most plants and animals, including humans, have two alleles of each gene, one from each parent. the order of bases in each allele is slightly different - they code for different version of the same characteristic. they’re represented using letters

Question 8

define genotype

Answer 8

the alleles an organism has, e.g. BB, Bb or bb for eye colour

Question 9

define phenotype

Answer 9

the characteristics the alleles produce, e.g. brown eyes

Question 10

define dominant

Answer 10

my kink

HA no not really the answer is –> an allele whose characteristic appears in the phenotype even when there’s only one copy

Question 11

define recessive

Answer 11

an allele whose characteristic only appears in the phenotype if two copies are present

Question 12

define codominant

Answer 12

alleles that are both expressed in the phenotype - neither one is recessive, e.g. the alleles for Hb

Question 13

define locus

Answer 13

the fixed position of a gene on a chromosome. alleles of a gene are found at the same locus on each chromosome in a pair

Question 14

define homozygote

Answer 14

an organism that carries two copies of the same allele, e.g. BB

Question 15

define heterozygote

Answer 15

an organism that carries two different alleles, e.g. Bb

Question 16

define carrier

Answer 16

a person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring

Question 17

occasionally, alleles show codominance - what does this mean?

Answer 17

both alleles are expressed in the phenotype, neither one is recessive. e.g. the human allele for sickle cell anaemia

Question 18

what is dihybrid inheritance and what does a dihybrid cross work out?

Answer 18

dihybrid inheritance is the inheritance of two characteristics which are controlled by different combinations of characteristics. you can use a dihybrid cross to look at how the two different genes are inherited at the same time

Question 19

How are some characteristics sex linked?

Answer 19

1) the genetic info for gender is carried on two sex chromosomes
2) in mammals, females have two X chromosomes and males have one X and one Y chromosome
3) a characteristic is said to be sex-linked when the allele that codes for it is located on a sex chromosome
4) the y chromosome is smaller than the x chromosome and carries few genes. so, most genes on the sex chromosomes are only carried on the x chromosome (called x-linked genes)
5) as males only have one X chromosome, they often only have one allele for sex-linked genes. so, because they only have one copy, they express the characteristic of this allele even if it’s recessive. this makes males more likely than females to show recessive phenotypes for genes that are sex-linked
6) genetic disorders caused by faulty alleles on sex chromosomes include colour blindness and haemophilia. the faulty alleles for both of these disorders are carried on the X chromosome - they’re called X-linked disorders

Question 20

What are X-linked disorders?

Answer 20

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

Question 21

Why are males more likely than females to express characteristics of recessive alleles that are sex-linked?

Answer 21

as males only have one X chromosome, they often only have one allele for sex-linked genes. so, because they only have one copy, they express the characteristic of this allele even if it’s recessive. this makes males more likely than females to show recessive phenotypes for genes that are sex-linked

Question 22

How are some autosomal genes linked?

Answer 22

1) autosomal is a name for any chromosome that isn’t a sex chromosome
2) genes on the same autosome are said to be linked - because they’re on the same autosome they’ll stay together during the independent assortment of chromosomes in meiosis I, and their alleles will be passed on to the offspring together. The only reason this won’t happen is if crossing over splits them up first
3) the closer together two genes are on the autosome, the more closely they are said to be linked. This is because crossing over is less likely to split them up
4) if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross
5) this allows you to use the predicted phenotypic ratio to identify autosomal linkage