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Biology > Topic 7: Genetics, Evolution and ecosystems > Flashcards

Topic 7: Genetics, Evolution and ecosystems Flashcards

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1
Q

What is a genotype?

A

The genetic makeup of an organism - all the alleles an organism has

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2
Q

What is a phenotype?

A
  • Observable characteristics of an organism

- The interaction between the expression of a dominant characteristic and the environment

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3
Q

What is a gene?

A

A sequence of DNA that codes for a polypeptide or a functional RNA (tRNA/rRNA)

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4
Q

What is an allele?

A

Different versions of the SAME GENE located at a locus on a chromosome

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5
Q

How many alleles of each gene do diploid cells usually have?

A

2 (One is dominant, other recessive)

Except on the Y chromosome (much shorter)

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6
Q

What does it mean when an organism is homozygous for a characteristic?

A

The allele is the same on both chromosome

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7
Q

What does hetrozygous mean?

A

The alleles (of the same gene) are different on each chromosome

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8
Q

What is a dominant allele?

A

An allele which is expressed all the time

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9
Q

What is a recessive allele?

A

The allele is not expressed in a heterozygote (ie. with a dominant allele)

It is only expressed in a homozyous recessive pair of chromosomes

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10
Q

What is it called when two alleles BOTH contribute to a phenotype?

A

Codominance

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11
Q

What is an example of codominance?

A

The ABO blood group system
A and B are both dominant to the allele O

Eg AO = Blood group A
AB = Blood group AB
Blood group O only occurs with OO alleles as

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12
Q

What is monohybrid inheritance?

A

The inheritance of a single gene

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13
Q

Who investigated monohybrid inheritance?

A

Mendel, using pea plants because of easily observable characteristics

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14
Q

When a tall plant is crossed with a short plant (both homozygous) what are the phenotypes of the F1 generation? (Tall is domonant)

A

Tall

Expalaination:
TT x tt

= Tt and Tt

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15
Q

If two heterozygous tall (Tt) plants are crossed, what is the outcome?

A

3 Tall plants, 1 short plant
(TT, Tt x2, tt)

A 3:1 ratio

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16
Q

What is the difference between observed and expected ratios is called?

A

A sampling error

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17
Q

What effect does the sample size have on genetic sampling errors?

A

Small populations will have a greater impact (more sampling error)

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18
Q

Why does a 9:3:3:1 or a 3:1 ratio rarely exactly exist (4 possibilities)?

A

A VERY COMMON GENETICS QUESTION

  1. Fertilisation is random
  2. Some alleles may be lethal
  3. Some populations may be small, so a more pronounced sampling error
  4. Some alleles may not be equally distributed among ‘viable gametes’, some die or are infertile
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19
Q

How can you tell if a tall plant is homozygous or heterozygous for a tall allele? (in monohybrid inheritance)

A
  • Self-fertile the tall plant
  • Back-cross using a short plant
    Observe the ratio of offspring
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20
Q

What do pedigree diagrams show?

A

Shows the ancestral relationship and transmission of genetic traits

21
Q

Why are pedigree diagrams useful?

A
  • Detects genetic diseases

- Predicts the chances of offspring being affected by a genetic disease

22
Q

How can you tell form a pedigree diagram whether an allele is dominant or recessive?

A
  • Look for parents that DO NOT have a characteristic but children who DO have it
  • Tells you that the allele is recessive
23
Q

What does dihybrid inheritance look at?

A

Looks at how two different characteristics, determined by two different genes on different chromosomes, are inherited

24
Q

What is an example of dihybrid inheritance?

A

Seed shape and colour in pea plants

Seed shape and colour are determined by two different genes on two different chromosomes

25
Q

What ratio of phenotype is expected in a dihybrid cross of homozygous parents (eg RrYy)?

A

9:3:3:1

26
Q

Why does a dihybrid cross of homozygous parents (RrYy) produce 4 different types of gamete?

A
  • The genes are on separate chromosomes
  • Homologous chromosomes arrange themselves on the cell equator at random in metaphase 1 of meiosis
  • Either Y and y or R and r can combine
27
Q

What are Mendel’s two laws?

A
  1. Law of segregation

2. Law of independent assortment

28
Q

What is the law of segregation (Mendel)?

A
  • Phenotypes are controlled by genes that occur in pairs

- Only one of each pair can be carried in the gamete

29
Q

What is the law of independent assortment (Mendel)?

A

Each member of a pair of alleles can combine randomly with either from another pair

30
Q

What is an autosome?

A

A chromosome that is not a sex chromosome (22 pairs in humans)

31
Q

What is autosomal linkage? What ratio of offspring would be expected?

A

Two or more genes are carried on the same autosome

3:1 ratio

32
Q

Do linked genes separate?

A

No, in accordance with Mendal’s laws

33
Q

In what stage of meiosis does crossing over occur?

A

In prophase 1

34
Q

What is codominance?

A

When both alleles are expressed in a phenotype - neither is dominant over the other

35
Q

What are the alleles of the offspring of two parents with heterogeneous for codominant alleles?

A

Parents both have alleles RW

Offspring have alleles RR, RW, RW, WW
So a 1:1:2 ratio of phenotypes

36
Q

Give an example of codominace in humans. What are the alleles and phenotypes?

A

Sickle cell anaemia

Normal haemoglobin governed by H allele, sickle cell governed by S allele

Neither is dominant, so heterozygous individuals (HS) have sickle cell trait which causes partial dysfunction of haemoglobin

37
Q

What is Epistasis?

A

Where the allele for one gene affects or masks the expression of another in the phenotype

38
Q

What is an example of Epistasis? What genes are masked?

A

Coat colour in mice

Gene A controls menalin DISTRIBUTION
Dominant allele A leads to black bands
Recessive allele a leads to uniform black

Gene B determines the expression of A because:
Dominant allele B leads to menalin production
Recessive allele b leads to no menalin produced

Thus, if Gene B is bb, then no pigment is expressed so no colour regardless of gene A expression (albedo)

39
Q

How many sex chromosomes do humans have?

A

A pair (2)

40
Q

What are the sex chromosomes in women?

A

XX

41
Q

What are the sex chromosomes in men?

A

XY

42
Q

What is special about the Y chromosome?

A

It is much shorter than the X chromosome

43
Q

Which sex chromosomes do male offspring inherit?

A

Y chromosome always from their father

Thus X chromosome is always one of their mothers

44
Q

What happens if a male offspring possesses a recessive allele on the non-homologous part of their X chromosome?

A

It is always expressed because the Y chromosome is much shorter, so there is no dominant allele

45
Q

What is the non-homologous part of an X chromosome (sex linkage)?

A

The portion of the X chromosome that has no equivalent gene (and thus allele) on the Y chromosome because the Y chromosome is much shorter

46
Q

The allele governing colour-blindness is sex linked. How can two ‘normal’ people produce colour-blind offspring?

A

The mother must be a carrier (as the recessive allele is on the X chromosome, which is always expressed in men because no equivalent dominant allele on the Y chromosome)

47
Q

Can females be colour-blind?

A

Yes (sex linked)

Female parent must be carrier or colour blind and the father must be too

Female offspring inherit on X chromosome form each parent

48
Q

What must you do when give the question: “What is the probability that the offspring will be male and _”?

A

Multiply the probability of being _ by the probability of being male (0.5)

Biology (10 decks)

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