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Biology A2 > Inheritance > Flashcards

Inheritance Flashcards

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1
Q

What is the genotype?

A
  • genotype is the genetic constitution of an organism.

- describes all alleles an organism possesses.

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2
Q

What is the phenotype?

A
  • Phenotype is the expression of genetic constitution

- and its interaction with the environment

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3
Q

What is a gene?

A
  • Genes are short sequences of nucleotides carried on a strand of DNA
  • occupying a specific locus on a chromosome
  • which codes for a specific polypeptide. (polypeptide may be an enzyme which determines a particular physical characteristic eg. fur colour)
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4
Q

What are alleles?

A
  • Alleles are alternative forms of the same gene
  • dominant, recessive or codominant
  • occupying the same specific locus on a chromosome.
  • may be many alleles of a single gene (leading to variation in the characteristic coded for)
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5
Q

What is a pure-breeding organism?

A

Pure breeding: individuals which are homozygous for a particular characteristic
- consistently give rise to homozygous offspring

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6
Q

What is a heterozygote?

A

Heterozygous –

  • 2 different alleles for a specific characteristic on each homologous chromosome.
  • Only the dominant allele for the characteristic will be expressed in the phenotype.
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7
Q

What is a homozygote?

A

Homozygous
- 2 identical alleles for a specific characteristic on each homologous chromosome:
• Homozygous dominant = 2 dominant alleles
• Homozygous recessive = 2 recessive alleles

RECESSIVE ALLELES ARE ONLY EXPRESSED WHEN BOTH ALLELES ARE RECESSIVE IN THE DIPLOID STATE eg. both homologous chromosomes have recessive alleles.

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8
Q

What are the 4 different types of alleles?

A
  • Multiple alleles occur when a gene has more than 2 allele forms
  • Dominant is the allele that is always expressed in the phenotype if present
  • Recessive is the allele that is never expressed in the phenotype unless in a diploid homologous state where both alleles are recessive (no dominant alleles).
  • Co-dominant is when 2 different alleles contribute to the phenotype expressed by the organism. When both alleles occur together, the phenotype will be a blend of both features.
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9
Q

Give an example of co-dominance with multiple alleles

A 
human blood groups: codominance with multiple alleles 
O type = O+O alleles
B type = B+B or B+O alleles
A type = A+A or A+O alleles
AB type = A+B alleles
  • An individual possessing the B allele (dominant)- with or without the O allele (recessive) - expresses blood type B
  • An individual possessing A and B alleles - express AB blood type
  • Both A and B proteins synthesised (co-dominance)
    In a diploid organism, the alleles at a specific locus may be either homozygous (same tt / TT) or heterozygous (different Tt / Tt)
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10
Q

What does ‘expressed’ mean?

A

‘Expressed’ means that the characteristic is coded for, transcribed and translated into a protein

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11
Q

What is the locus?

A
  • Locus is the position of a gene on a particular DNA molecule.
  • Only one allele occupies the same ‘loci’.
  • Diploid organisms have homologous chromosomes (pairs) so contain 2 different alleles (one at each loci)
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12
Q

What are dominant alleles?

A

Dominant is the allele that is always expressed in the phenotype if present

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13
Q

What are recessive alleles?

A

Recessive is the allele that is never expressed in the phenotype
- unless in a diploid homologous state where both alleles are recessive (no dominant alleles).

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14
Q

What are co-dominant alleles?

A

Co-dominant is when 2 different alleles contribute to the phenotype expressed by the organism.
- When both alleles occur together, the phenotype will be a blend of both features.

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15
Q

What are multiple alleles?

A

Multiple alleles occur when a gene has more than 2 allele forms

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16
Q

What are multiple alleles?

A

Multiple alleles occur when a gene has more than 2 allele forms
However, as there are only 2 chromosomes in a homologous pair, there are only 2 gene loci – so an organism can only possess 2 forms at once.

17
Q

what is monohybrid inheritance?

A

Monohybrid = inheritance of a single characteristic

18
Q

what is monohybrid inheritance?

A

Monohybrid = inheritance of a single characteristic

19
Q

what is dihybrid inheritance?

A

Dihybrid = inheritance of two characteristics: determined by 2 different genes on different chromosomes.

20
Q

What should a genetic diagram contain ‘rules’ ?

A 
o	Parent phenotypes
o	Parent genotypes
o	Parent gamete genotypes
o	Offspring genotypes 
o	Offspring phenotypes
o	Punnett square
o      Probability of characteristics
21
Q

What is the expected ratio for a monohybrid cross?

Dihybrid?

A

Theoretically, the ratio in monohybrid crosses is always 3:1
- 3 with dominant alleles BB, Bb, Bb and 1 with bb recessive alleles

dihybrid = 9:3:3:1

22
Q

Why is the expected ratio for a monohybrid cross different to the actual ratio?

A
  • expected 3:1 ratio is rarely achieved due to chance.
  • does not tell us the actual ratio, but the most likely one.
  • For gametes, it is chance that determines which gametes fuse with which.
  • or codominance may be to blame!
  • The larger the sample, the more likely it is that the actual results will match the theoretical results
  • It is important to use large samples in genetic crosses to ensure that:
    • representative results are obtained
    • experiment is replicable to assess reliability
    • free from statistical errors
23
Q

Why must we use large samples in genetic crosses?

A
  • larger the sample, the more likely actual results will match theoretical results
  • large samples in genetic crosses ensures that:
    • representative results are obtained
    • experiment is replicable to assess reliability
    • free from statistical errors
24
Q

How do we display the 2 characteristics in dihybrid inheritance?

A

Capital letters for both characteristics because lower case would suggest that one characteristic is dominant to the other.

  • suitable capital letter for the constant characteristic eg. C for ‘colour’
  • suitable capitals for alternatives of alleles eg. R for red and W for white
  • alternatives are written as ‘superscripts’ above the constant letter
25
Q

What is Mendel’s law of independent assortment?

A

Mendel’s law of independent assortment: as chromosomes arrange randomly at the equator of the cell, any one of the 2 alleles in a pair can combine randomly with either of another pair.

26
Q

What is sex-linkage?

A

Any gene carried on the X or Y chromosome is said to be sex linked
• The female X chromosome is much longer than the male Y chromosome
• This means that most of the X chromosome doesn’t have an equivalent portion on the Y chromosome

  • This means that recessive alleles on this portion of the X chromosome will be more likely to be expressed; because of this recessive phenotypes are more likely to be present in men.

Haemophilia is for this reason almost entirely only present in men and not women

  • Males can only obtain the disease from their mother as they do not receive a Y chromosome from the father
  • Males cannot pass the disease on to their sons but they can to their daughters
  • characteristics controlled by recessive alleles on the non-homologous portion of the X chromosome will appear more frequently in males because there is no homologous portion on the Y chromosome that might be dominant to mask the recessive allele
27
Q

What is an X-linked disorder?

A

An X-linked genetic disorder is a disorder caused by a defective gene on the X chromosome

One example is haemophilia, in which blood clots slowly and there may be internal bleeding

  • caused by a recessive allele with an altered sequence of DNA nucleotide bases (thus, different tertiary structure of hydrogen, ionic and sulfuric bonds)
  • code for a non-functional protein.
  • unable to produce functional protein required in clotting

The production of this functional protein by genetically modified organisms means that haemophiliacs can be given the modified protein, allowing them to lead near-normal lives
As they are linked to the X chromosome, they are shown attached to the X chromosome (XH and Xh)
There is no equivalent allele on the Y chromosome as it does not carry the gene for producing clotting protein

As the defective allele that does not code for the clotting protein is linked to the X chromosome, males always inherit the disease from their mothers.
If their mother does not suffer from the disease, she may be heterozygous (XH Xh) and be a carrier of the allele without showing any signs of the disease in their phenotype because they possess one dominant H allele, leading to the production of enough functional clotting protein so that symptoms are masked.

28
Q

What are pedigree charts?

A

Pedigree charts:
A useful way to trace the inheritance of sex – linked characters is with a pedigree chart. In these:
A female is represented by a square
A male is represented a circle
Shading within the shape represents the presence of a certain character A dot within a circle indicated a normal phenotype shading within either shape indicates the presence of a character, such as haemophilia, in the phenotype

29
Q

Why do males inherit sex-linked characteristics more frequently than women?

A
  • Any gene carried on the X or Y chromosome is said to be sex linked
    • The female X chromosome is much longer than the male Y chromosome
    • This means that most of the X chromosome doesn’t have an equivalent portion on the Y chromosome
  • This means that recessive alleles on this portion of the X chromosome will be more likely to be expressed because there is no homologous portion on the Y chromosome that might be dominant to mask the recessive allele
30
Q

What is the Hardy Weinberg principle?

A
  • number of times an allele occurs in a population is called the allelic frequency
  • HW principle is a mathematical model used to calculate allelic frequency
31
Q

What assumptions do the HW principles make?

A

calculate allelic frequencies provided that:
• No mutations arise
• The population is isolate (no immigration emigration)
• There is no selective breeding
• The population is large
• Mating within the population is random

32
Q

What assumptions do the HW principles make?

A

calculate allelic frequencies provided that:
• No mutations arise
• no immigration or emigration
• There is no selective breeding advantages
• The population is large
• Mating within the population is random

33
Q

How do multiple alleles of a gene arise?

A

mutations;

which are different / at different positions in the gene;

Biology A2 (3 decks)

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