Inheritance

Question 1

gene

Answer 1

unit of inheritane located at a particular locus of a choromosome. it is a specific DNA nucleotide sequence which codes for RNA or a polypeptide

Question 2

Locus

Answer 2

The specific location of a gene on a chromosome.

(plural: loci)

Question 3

Allele

Answer 3

• An alternative form of a gene at a particular gene locus.
• Responsible for determining contrasting traits of the same character.
• All alleles of a gene determine the same character, but each has a unique DNA nucleotide sequence, which may result in different phenotypes.
• Alleles occur in pairs in a diploid cell although only one of the pair is represented in a gamete

Question 4

dominant vs recessive alleles

Answer 4

dominant alleles:
- Produce their effects in both homozygous and heterozygous condition
- one copy of the allele is sufficient to cause the organism to express the phenotype it encodes, e.g. allele T causes the plant to be tall in either TT or Tt condition.
- A dominant allele masks the influence of the recessive allele.
- An organism homozygous for a dominant allele is known as homozygous dominant.

Recessive alleles:
- Produce their effects only in homozygous condition, e.g. allele t causes the plant to be short only in the tt condition.
- An organism homozygous for a recessive allele is known as homozygous recessive.

Question 5

codominace

Answer 5

• phenomenon which both alleles are equally expressed in the phenotype of the heterozygote.
• heterozygote simultaneously expresses the phenotypes of both types of homozygotes

eg in cows, heterozygote with roan coat consist of a mixture of red and white hairs

Question 6

homozygous vs heterozygous

Answer 6

Homozygous:
- Condition in which the alleles of a gene pair in diploid condition are identical, e.g. TT or tt, where T is the dominant allele for tall stem and t is the recessive allele for short stem.
- An organism with this condition is known as a homozygote and referred to as true or pure breeding. All gametes produced by this organism will carry the same allele.

Heterozygous
- Condition in which the alleles of a gene pair in diploid condition are different, e.g. Tt.
- An organism with this condition is known as a heterozygote. 50% of its gametes will carry one allele while the other 50% will carry the other allele.

Question 7

phenotype vs genotype

Answer 7

Genotype:
- The complete genetic makeup / allelic composition of an organism.
- The term is also commonly used in reference to the paired alleles carried by an organism that give rise to a phenotype.

Phenotype
- The physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment.

Question 8

linkage

Question 9

what is the link between genotype and phenotype?

Answer 9

• Alleles, being alternative or different forms of a gene, are transcribed to form mRNA and translated into different polypeptide chains and proteins which may affect different metabolic pathways resulting in the formation of different phenotypes
• Each allele thus specifies or codes for the specific 3D conformation of a particular protein which directly determines a trait
• A genotype is the genetic makeup of an organism, which refers to the paired alleles that produces a phenotype which is a measurable or distinctive character

Question 10

monohybrid inheritance

Answer 10

it is the inheritance of a single character of contrasting traits (e.g. tall vs short stem)
- character is controlled by a single gene (a pair of alleles)

Question 11

what r mendel’s postulations

Answer 11

1. alleles come in pairs
   - In the monohybrid cross involving tall and dwarf plants, a specific allele, T and t exists for each trait respectively.
   - Each diploid individual receives one allele from each parent.
2. dominance or recessiveness
   - in Tt, recessive allele is masked by dominant allele, and is only only able to express itself in the outward appearance of the plant
   in the absence of the dominant allele,
3. segregation:
   Mendel’s first law of segregation: During formation of gametes, paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood

Question 12

what is a test cross (monohybrid inheritance)

Answer 12

purpose: involves crossing organism of the dominant phenotype but unknown genotype with homozygous recessive individual, phenotype of the offspring indicates genotype of the organism tested

• if organism displaying dominant phenotype is homozygous, all offspring will express the dominant phenotype
• if organism if heterozygous, about ahlf the offspring will express the dominant phenotype while the other half expresses the recessive phenotype in a 1:1 ratio

Question 13

what is test cross (dihybrid inheritance)

Answer 13

crossing organism that expresses two dominant traits but unknown genotype with an individual which is homozygous recessive for both gene loci (double homozygous recessive)

Question 14

what is a lethal gene

Answer 14

lethal allele: one that causes death is organism is homozygous for allele

dominant lethal gene:
expressed in homozygotes and heterozygotes, rarely detected due to rapid elimination from both populations

recessive lethal gene
eg Coat colour in mice is linked to a lethal gene
- mutant allele Y is dominant to wild type agouti allele y
- gene is recessive allele as only with 2 YY, it is lethal
- heterozygous: yellow coat
- cross breeding yellow mice (Yy) –> 2 yellow fur (Yy): 1 agouti fur (yy) cuz homozyogus yellow (YY) mice die

Question 15

what is a dihybrid cross

Answer 15

genetic cross involving two characters in which the parents possess different forms of each character. the two characters are controlled by two genes (2 pairs of alleles) located on two gene loci on two different chromosomes (unlinked genes)

Question 16

what is mendel’s second law of independent assortment

Answer 16

segregation of one pair of alleles is independent of segregation of other pairs

this means:
- each F1 gamete receives either a R or r allele AND a Y or y allele
- all four combinations (RY, Ry, rY, ry) formed with equal probabilities

note: independent assortment occurs only if genes are not located on same chromosome (aka not linked genes)

Question 17

how do 23rd chromosome differ in males and females and how do they determine sex?

Answer 17

females are homogametic: produces one type of gamete, all carry X chromosome
males are heterogametic: produces two types of gametes, half carry X, half carry Y

in offspring, sex is determined by sperm that fertilises egg cell:
egg cell may be fertilised by sperm carrying an X chromosome: zygote has two X chromosomes, female
egg cell fertilised by sperm carrying Y chromosome: zygote has XY, male

Question 18

what are sex-linked genes / X-linked genes

Answer 18

genes located on sex chromosomes
X chromosome contains many loci that are required on both sexes, hence genes are mostly located on X chromosme

Question 19

why do human sex-linked inheritance mainly affect males?

Answer 19

As a male has only one sex-linked gene locus (1 copy of X chromosome instead of 2 in females), he is hemizygous (half) for every sex-linked locus
* single X chromosome, whatever allele present on the X chromosome of males will be directly expressed in the phenotype, whether or not if the
allele is dominant or recessive

Question 20

what are some principles regarding sex linked inheritance

Answer 20

• affected father will transmit the recessive X-linked allele to all daughters but not to any sons because his son will inherit father’s Y chromosome only and X chromosome from mother.
• If the mother is homozygous dominant, the daughters are heterozygous and are carriers of the mutation
• carrier female x normal male: 50% chance that each daughter will be a carrier, 50% chance that each son will have the disorder
• carrier female x affected male: 50% chance that each child born to them will have the disorder, regardless of sex

Question 21

how does a reciprocal cross work

Answer 21

reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome (X-linked) or on an autosomal chromosome.
A reciprocal cross is a pair of crosses in which the traits of the two parents are reversed.

if results are non-identical in the two crosses, it involves X linked traits, whilst if resutls are identical, cross involves autosomal traits

Question 22

why do X-linked reciprocal crosses reuslt in non identical results?

Answer 22

• the male transmits the X chromosome only to his female offspring,
• while the female transmits an X chromosome to both male and female offspring.
• Since the male parent does not transmit the X chromosome to his sons, he does not contribute to
  the appearance of their X-linked phenotypes.

Question 23

what are some characteristics of autosomal recessive inheritance

Answer 23

Recessive because:
1. If both parents are affected (aa x aa), all children should be affected
2. In most cases, when unaffected people mate with affected individuals, all children are unaffected.
3. When at least 1 child is affected (means unaffected parent is heterozygous), approximately half the children should be affected.
4. Unaffected parents (heterozygous at gene loci) can produce affected individuals.
5. Trait often skips generations.

Autosomal because:
1. Both males and females are affected with equal probability.

whilst for dominant:
- if both parents are affected (Aa / AA), not all children will be affected (due to possibility of aa genotype which is not affected)

Question 24

Characteristics of an autosomal dominant inheritance

Answer 24

Dominant because:
1. Unaffected parents (aa) should not have affected children.
2. Trait should not skip generations, i.e. affected individuals must have affected parent(s).
3. When an affected heterozygous (Aa) individual mates with an unaffected (aa) individual, 50% of their offspring should be affected (also shows that affected individual is heterozygous).

Autosomal because:
1. Both males and females are affected with equal probability.

Question 25

Characteristics of a X-linked recessive inheritance

Answer 25

X-linked because: 1. **Most affected individuals are males.** 2. Affected mothers produce affected sons with 100% chance. 3. Affected females are the result of a mating between affected fathers and affected or carrier mothers. 4. Approximately half of the sons of carrier females should be affected. Recessive because: 1. If fathers are not affected, daughters will not be affected but may be carriers. ## Footnote for x linked recessive: Affected mothers produce affected sons with 100% chance. x linked domnant: affected mothers produce affected sons with 50% chance autosomal: probability of male/female offspring being affected is same

Question 26

x-linked dominant inheritance

Answer 26

X-linked because: 1. Affected mothers produce affected sons (only 50% chance). 2. Affected females are the offspring of affected mothers or fathers. 3. All daughters, but none of the sons, of an affected father are affected, assuming mother is normal Dominant because: 1. The trait does not skip generations. 2. Approximately half of the children of the affected heterozygote females are affected.

Question 27

what are linked genes

Answer 27

genes that control different characters and are situated on same chromosome at different loci