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Biology🧬 > Inheritance👨‍👩‍👧‍👦 > Flashcards

Inheritance👨‍👩‍👧‍👦 Flashcards

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1
Q

What is a gene?

A

A specific length of DNA which occupies a position on a chromosome called a locus

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2
Q

What are polygenic traits?

A

Most characteristics are determined by several different genes interacting

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3
Q

What is a locus?

A

A particular position of a gene on a chromosome

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4
Q

What are alleles?

A
  • Alternative forms of the same gene

* May alter the way in which a particular characteristic is expressed

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5
Q

What is a dominant allele?

A

If this allele is present, the individual has the characteristic

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6
Q

What is a recessive allele?

A

•Individual only has the characteristic if both chromosomes in the homologous pair have the recessive allele
-i.e. if the individual is homozygous recessive
•Cystic fibrosis is an example of a genetic condition that is caused by a recessive allele

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7
Q

What does homozygous mean?

A

•Both chromosomes in a homologous pair have the same allele

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8
Q

What is homozygous dominant?

A

Both chromosomes have the dominant allele

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9
Q

What is homozygous recessive?

A

Both chromosomes have the recessive allele

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10
Q

What does heterozygous mean?

A
  • The alleles on the chromosome of the homologous pair are different
  • Individual has a dominant and a recessive allele for a particular characteristic and could be said to be ‘carrying’ the recessive characteristic
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11
Q

What does carrying mean?

A

Characteristic is not shown by the individual but could be passed on to their offspring

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12
Q

What is a genotype?

A

The alleles possessed by an organism

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13
Q

What is a phenotype?

A

The physical characteristics of an organism controlled by the genotype

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14
Q

What is monohybrid inheritance?

A
  • The inheritance of a pair of contrasting characteristics controlled by two alleles at a single locus
  • An example is Gregor Mendel’s pea shoot experiments
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15
Q

Step-to-step of a genetic cross

A
  • Show genotypes of the parents
  • Represent the possible gametes from each parent and draw a circle around them
  • Construct a Punnett square and put the possible gametes from one parent in the first vertical column and the possible gametes from the other parent in the first horizontal row
  • Carry out the genetic cross, combining the gametes, and write the genotype formed in the appropriate square
  • Count the different genotypes and phenotypes and represent them as a percentage or a ratio
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16
Q

What is a test cross/back cross?

A
  • Used to work out whether an individual with a dominant characteristic is homozygous dominant or heterozygous
  • Involves breeding the individual of unknown genotype with an individual that is homozygous recessive
  • By looking at the phenotypes of the offspring, it is then possible to determine whether the unknown individual was homozygous dominant or heterozygous
  • If breeding the individual of unknown genotype with a homozygous recessive individual produces any offspring expressing the recessive allele, the unknown individual must be heretozygous
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17
Q

Expected phenotypic ratios in offspring

A

Crossing two heterozygous you will always get the ratio 3:1

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18
Q

What is codominance?

A
  • Some alleles are codominant

* Both alleles are expressed in the phenotype

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19
Q

Example of codominance - blood groups in humans

A
  • I^AI^A only have A antigens in their blood, while humans who are I^BI^B only have B antigens
  • Heterozygous humans (I^AI^B) have both antigens in their blood as both alleles are expressed
  • This means if one parent is I^AI^A and the other parent is I^BI^B all their offspring will be I^AI^B
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20
Q

What are heterosomes?

A
  • Human sex is determined by a pair of sex chromosomes called X and Y
  • Called heterosomes as they don’t look alike
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21
Q

What are autosomes?

A

All other chromosomes (other than heterosomes) are called autosomes

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22
Q

What is sex-linkage?

A
  • Genes that are located on one or other of the sex chromosomes are said to be sex linked
  • A sex linked condition is one that is controlled by an allele on the X or Y chromosome
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23
Q

Why are most sex linked conditions carried on the X chromosome?

A

Due to the small size of the Y chromosome

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24
Q

Examples of sex linked conditions

A
  • Haemophilia

* Duchenne muscular dystrophy

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25
Q

What is haemophilia? (Sex linked genetic condition)

A
  • Caused by a recessive allele found on X chromosome, represented by X^h
  • As on X chromosome, female haemophiliac must be homozygous recessive (X^hX^h) while a Male haemophiliac has only one recessive allele (X^hY)
26
Q

Why is it possible for a female carrier and a healthy male to have a haemophiliac son but not a haemophiliac daughter?

A
  • X chromosome from father would be X^H as the father is not a haemophiliac
  • The mother of a female haemophiliac would have to be either a carrier or have the condition -her father would have to be a haemophiliac
27
Q

What is dihybrid inheritance?

A
  • Involves the inheritance of two genes not carried on the same chromosome
  • Each gene has 2 alleles
  • Mendel studied this in seeds of pea plants
28
Q

What leads to different genotypes in the offspring of dihybrid inheritance?

A

Independent assortment

29
Q

What is the typical expected phenotype ratio in dihybrid inheritance?

A

9:3:3:1

30
Q

What is linkage?

A

•Alleles on the same chromosome will be inherited together
•This is because when the chromosome passes into a gamete, the alleles are all present
•Therefore, if that gamete is fertilised, all those alleles will be incorporated into the genome of the offspring
•Do not follow the expected Mendelian ratios
-non-Mendelian patterns of inheritance

31
Q

How is linkage further complicated?

A
  • Crossing over swaps genes between chromosomes

* This means the linkage can be changed

32
Q

What are recombinant offspring?

A

Individuals with different phenotypes to parents

33
Q

Recombinant offspring in linkage

A

•Occurrence of recombinant offspring shows that linkage was not complete due to crossing over during prophase 1 of meiosis separating the genes

34
Q

Process of recombinant formation in linkage

A
  • Homologous chromosomes pair up during prophase 1
  • Chiasma formation
  • Homologous chromosomes separate during anaphase 1
  • Chromatids separate during anaphase 2, forming 4 different gametes
35
Q

Down’s syndrome

A

Caused by an extra chromosome 21 - trisomy 21

36
Q

How can trisomy be achieved in a zygote?

A
  • Due to non-disjunction in anaphase 1 of meiosis

* Chromosomes fail to separate

37
Q

Why was Gregor Mendel’s research useful?

A
  • Large numbers of data so statistically valid
  • Meticulous records
  • Analysed data - conclusions made based upon evidence
38
Q

Particulate inheritance

A

Mendel thought characteristics were passed on by particles

39
Q

What were Mendel’s laws?

A
  • Law of segregation

* Law of independent assortment

40
Q

The law of segregation?

A
  • Every organism possesses a pair of alleles for any particular characteristic
  • Each parent passes a copy of only one of those alleles to their offspring
  • Whichever of the alleles is dominant affects the phenotype of the organism
41
Q

The law of independent assortment

A
  • States that genes are passed independently of other genes from parents to offspring (the selection of a gene is not affected by the selection of other genes)
  • The genes assort independently during gamete formation
42
Q

Multiple alleles

A

More than two alleles

43
Q

Incomplete linkage

A
  • Genes are on the same chromosome but are far apart

* Crossing over may occur between them during prophase 1

44
Q

Controlling gene expression

A
  • DNA can be modified post-replication
  • This does not change the DNA base sequence but changes the ability of a gene to be transcribed during protein synthesis
  • The gene may be suppressed or expressed more
45
Q

Epigenetics

A

The control of gene expression by factors other than changes in the DNA sequence

46
Q

Epigenetic mechanisms

A

•DNA methylation - addition of methyl groups - prevents those bases being recognised and reduces the ability of that gene to be expressed
•Histone acetylation - histone proteins used to organise the DNA in a chromosome can also be modified
-If the histone is more tightly coiled, this can prevent gene expression
-If the histone is less tightly coiled, this can increase gene expression
•Different epigenetic modifications across different cells and tissues result in the expressions of the same gene in different parts of an organism

47
Q

Duchenne muscular dystrophy

A
  • An example of a condition that is sex linked
  • The faulty gene is only carried on the X chromosome, so muscular dystrophy has the same inheritance pattern as haemophilia
48
Q

What are mutations?

A
  • Random change in the volume, arrangement or structure in the DNA of an organism
  • Mutations can either affect a particular gene or a whole chromosome
  • Most mutations occur during crossing over in prophase 1 and through non-disjunction in anaphase 1 and anaphase 2
49
Q

What is addition? (gene mutations)

A

Nucleotides are inserted into the DNA sequence

50
Q

What is deletion? (gene mutations)

A

Nucleotides are removed from a DNA sequence

51
Q

What is substitution? (gene mutations)

A

A section of nucleotides is swapped for other nucleotides

52
Q

What is inversion? (gene mutations)

A

A section of nucleotides is reversed

53
Q

What is duplication? (gene mutations)

A

A section of nucleotides is duplicated

54
Q

What could a mutation result in?

A
  • Produce a change in DNA sequence of a gene
  • During transcription, this change will be incorporated into the mRNA molecule formed
  • Could lead to the polypeptide formed in translation having an incorrect amino acid sequence - primary structure would be incorrect
  • Could lead to a change in the folding of the polypeptide and therefore to a change in the tertiary structure of the protein
  • An alteration in the tertiary structure of a protein could render it non-functional
55
Q

Sickle cell anaemia - gene mutation

A
  • Mutation occurs in one nucleotide (T replaced by A) in one gene
  • In sickle cell anaemia, the RBC have a rigid sickle shape
  • Change in shape and lack of flexibility can lead to a variety of complications, including blockage of capillaries, which cuts off the blood supply to organs and results in organ damage
56
Q

Sickle cell anaemia - genetic terminology

A

•Allele that causes sickle cell anaemia is recessive - s
•Therefore, only individuals who are homozygous for this allele have the condition
•Heterozygous individuals have what is known as sickle cell trait, which doesn’t usually cause any symptoms
-however, makes individual resistant to malaria
-encourages sickle cell allele to be passed onto areas where malaria is prevalent as the heterozygous phenotype has a selective advantage

57
Q

Change in structure (chromosome mutations)

A

The structure of a chromosome can alter because of errors during crossing over

58
Q

Changes in number (chromosome mutations)

A

•Errors in meiosis can result in a gamete receiving the incorrect number of chromosomes
•The embryo formed from this gamete will then also have an incorrect number of chromosomes
•E.g. trisomy 21
-extra copy of chromosome 21
-Down syndrome

59
Q

Changes in sets of chromosomes (chromosome mutations)

A
  • A gamete can have a whole extra set of chromosomes

* This is known as polyploidy

60
Q

Mutation frequency

A

It is important to remember that mutagens don’t always cause mutations - they increase the frequency of mutation

61
Q

Mutation rates

A

•Normally very low
•In general, organisms with short life cycles and more frequent meiosis show a greater rate of mutation
•Mutagens increase the chance of a mutation occurring
-e.g. ionising radiations (UV light and X-rays) and certain chemical (polycyclic hydrocarbons in cigarette smoke)
•A mutagen that increases chance of developing cancer is a carcinogen
•Mutations in proto-oncogenes can result in them becoming oncogenes, leading to cells dividing uncontrollably and the development of cancer

62
Q

Explain why genes found in the sex chromosome pair have a pattern of inheritance that is different from genes found on other chromosomes pairs

A
  • X chromosome carries genes/loci not present on the Y chromosome
  • Males only have one copy/allele of some genes
  • If only one allele inherited it will be expressed

Biology🧬 (24 decks)

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