3.3.6 Inheritance Flashcards Preview

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Flashcards in 3.3.6 Inheritance Deck (54)
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Define gene

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


Define Genotype

The genetic constitution of an organism

(i.e. the alleles an organism has)


Define Phenotype

The expression of the genetic constitution and its interaction with the environment

(i.e. an organism’s characteristics)


Define Allele

A different version of a gene


Explain what is meant by a recessive allele

Only expressed in phenotype when 2 alleles are present


Explain what is meant by a dominant allele

Is always expressed in the phenotype


Explain what is meant by co-dominant alleles

Both alleles are expressed in the phenotype


Define Homozygous

An organism that carries 2 copies of the same allele


Define Heterozygous

An organism that carriers 2 different alleles


Define Carrier 

Person carrying an allele which isn't expressed in phenotype but can be passed on to offspring


Define Locus

Fixed position of gene on a chromosome

(Alleles of gene are found at same locus on each chromosome in a pair)


Define Autosome 

Any chromosome that is not a sex chromosome


Define Allosome 

Sex chromosome


What is monohybrid inheritance?

Inheritance of a characteristic controlled by a single gene


Draw a monohybrid cross showing how wing length is inherited in fruit flies when the parent's genotypes are NN x nn. Include the phenotypes and phenotypic ratio.

N - normal wings allele

n - vestigial (little) wings allele


Draw genetic diagram when the paren'ts genotypes are HNHS x HNHS (i.e. crossing 2 parents with sickle-cell trait (heterozygous))

HN = normal haemoglobin 

HS = sickle haemoglobin


In the ABO blood group system in humans there are 3 alleles for blood type: 

  • IO is the allele for blood group O 
  • IA is the allele for blood group A
  • IB is the allele for blood group B

Allele IO is recessive. Alleles IA and IB are codominant - people with genotype IAIB will have blood group AB.

Draw a genetic diagram that shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B.


What do dihybrid crosses show?

Show how 2 genes are inherited are once


Draw a dihybrid cross diagram for when the both parents' gentotype is RrYy. Include the phenotype and the phenotypic ratio.

  • R - round seed
  • r - wrinkled seed
  • Y - yellow seed
  • y - green seed 



When are characteristics said to be sex-linked?

When the allele that codes for a characteristic is located on a sex chromosome


Most genes on sex chromosomes are only carried on ___ chromosomes


(X-linked genes)


Explain why males are more likely than females to show recessive phenotypes for genes that are sex-linked

  • Males only have 1 X = 1 allele for sex-linked genes
  • ∵ have only 1 copy, express characteristic of allele even if its recessive


Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome 

Given that the female parent is a carrier and the male is unaffected, draw monohybrid diagram (n = faulty allele for a colour vision)


What are autosomal genes?

Genes located on the autosomes


Genes on the same autosome are said to be ____



Why are autosomal genes said to be linked?

  • ∵ they're on the same autosome, they'll stay together in independent segregation of chromosomes in meiosis I
  • And their alleles will be passed on to offspring together

(Only doesn't occur if crossing over splits them up 1st)


The closer together 2 genes are on autosome, the more ...

closely they're linked


Why is that the more closer together 2 genes are on autosome, the more closely they're linked?

∵ reduces the chance of genes being separated during crossing over


What happens to the phenotypic ratio expected in offspring when 2 gene are autosomally linked?

  • It changes - won't get the phenotypic ratio you expect in offspring of cross
    • e.g. Dihybrid cross between 2 heterozygous parents = 9:3:3:1 ratio in offspring
    • Instead phenotypic ratio more likely to be like monohybrid cross between heterozygous parents (3:1)

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