Inheritance (C2)

Question 1

What is an allele?

Answer 1

• alleles as different forms of the same gene
e.g GENE for hair colour - different ALLELE for brown hair and another allele for blond hair (one gene, different alleles)
• occupy similar locus on homologous chromosomes
• allele combinations;
heterozygous (diff alleles for a given gene - Bb)
homozygous dominant (same two dominant allele - BB)
homozygous recessive (same two recessive allele - bb)

Question 2

What is a gene and three main characteristics? locus?

Answer 2

GENE - a sequence of DNA bases that occupy a specific locus on a chromosome
1. code for production of specific polypeptides
2. can mutate
3. can separate and combine
LOCUS - position of gene on a chromosome

Question 3

What are genotypes and phenotypes?

Answer 3

genotype - all of the alleles an individual contains

phenotype - observable characteristics, how the alleles are expressed

Question 4

Define Mendelian monohybrid inheritance? Mendel’s first ‘law of inheritance’?

Answer 4

the inheritance of a trait controlled by a singular gene i.e plant height, colour or texture in the case of Mendel’s peas

first ‘law of inheritance’ - (applies for non linked genes/ genes on separate chromosomes) the characteristics of an organism are determined by alleles which occur in pairs, only one allele of a pair is present in each gamete

Question 5

What is incomplete dominance, why is it a non-mendelian trait? example?

Answer 5

• where heterozygous (Bb) offspring produce a phenotype intermediate of the parental phenotypes (two diff dominant genotypes)
• heterozygous phenotype is somewhere between/a blend
e.g snapdragon flowers; flowers can be red, white or pink - a red dominate (RR) parent and white dominant parent (WW) can produce pink offspring (RW) - a blend of red and white rather than one dominant gene being expressed thus not following the Mendelian law

Question 6

What is codominance, why is it a non-mendelian trait? example?

Answer 6

• where both individual alleles for a gene are expressed in a heterozygote
e.g cattle can have red, white or roan (red and white) coats - codominance results in red and white speckles, neither one dominant trait is expressed thus not upholding Mendelian law

Question 7

What are test crosses and why may they be carried out?

Answer 7

• a method used in genetics to determine whether a particular dominant characteristic expressed in an organism is controlled by;
- one dominant allele (heterozygous) OR
- two dominant alleles (homozygous dominant)
ie a brown haired cow could be BB or Bb we do not know unless we cross with a RECESSIVE phenotype

Question 8

What do pure breeding individuals mean? genotype?

Answer 8

two organisms of the SAME phenotype that will only produce offspring of the SAME phenotype when bred together THEREFORE, the organisms must be homozygous

*essentially pure bred organisms are homozygous (BB, bb)

Question 9

What is Mendelian dihybrid inheritance, Mendel’s ratio? how are recombinants produced?

Answer 9

• involves the inheritance of two unlinked genes (different chromosomes) e.g a round yellow pea or a green wrinkle pea - two unlinked genes
• 9 : 3 : 3 : 1
• independent assortment in meiosis metaphase I and II of these non-linked genes produce RECOMNINANTS - either pair of alleles may randomly combine with another this is MENDEL’S SECOND LAW (law of independent assortment)

Question 10

Why and when do you use Chi2 test?

Answer 10

• used to determine if the results of a genetic cross (OBSERVED values) are significantly different to EXPECTED results - other influencing factors
  OR
  whether the differences are due to chance alone

Question 11

What is the null hypothesis used for Chi2? when do we accept/reject? probability? degrees of freedom?

Answer 11

there is NO SIGNIFICANT DIFFERENCE between the observed and the expected
ACCEPT: if the Chi2 value is LESS than the critical value we accept the null hypothesis - any difference is down to chance
REJECT: if the Chi2 value is MORE than the critical value we reject the null hypothesis - difference must be influenced by other factors

• always use probability of 0.05/ 5%
• degrees of freedom - one less than number of categories

Question 12

Chi2 formula and how to calculate expected values?

Answer 12

chi2 = total of (O-E)2
———–
E

• expected would be the total of observed applied to Mendel’s ratio - 9:3:3:1
  e.g 9+3+3+1 = 16
  40 total
  40/16 = Ans x9, x3, x3, x1

Question 13

What are autosomes? sex chromosomes?

Answer 13

AUTOSOMES: any chromosome that is NOT a sex chromosome
SEX CHROMOSOME: a chromosome concerned in determining the sex of an organism, typically one of two kinds ( i.e x and y)
• in humans - they are the same in one sex (xx - females) and dissimilar in the other (xy - males)

• humans have 23 pairs of chromosomes; first 22 pairs are autosomes the last pair are the sex chromosomes

Question 14

What is sex-linage inheritance? male inheritance?

Answer 14

• sex-linkage in organisms with X and Y sex chromosomes, where the inheritance of a gene is present only on the X chromosome
(the y is much smaller and thus carries fewer genes)
• if an allele is carried on the x chromosome of a male, it will be expressed in the phenotype due to lack of correspondence of y allele
• sex-linked inherited diseases;
haemophilia
duchenne muscular dystrophy

Question 15

What is haemophilia? symptoms? genotypes and phenotypes?

Answer 15

• a sex-linked recessive condition where an individual cannot produce enough of one particular blood clotting protein
• slow, constant bleeding as a result of blood not being able to clot
• H - healthy blood clotting (dominant)
h - haemophilia (recessive)
• xHxH - healthy female, xHxh - carrier female, xhxh - haemophilia female, xH y healthy male, xh y - haemophilia male

Question 16

What is Duchenne muscular dystrophy? symptoms? genotypes? number of children?

Answer 16

• caused by a sex-linked recessive allele of the dystrophin, a protein in skeletal and muscular fibres, the gene codes for the protein stabilising cell membranes of muscle fibres
• progressive weakening and ‘wasting’ of muscles
• D - healthy protein
d - mutant protein
• may be a low number of children, affected individuals may not reach reproductive age or decide not to conceive offspring thus not pass on disease

Question 17

What is meant by linkage? do Mendel’s principles apply?

Answer 17

• genes on the same chromosome are linked (usually inherited together)
• chromosomes contain a linear sequence of genes which are all linked THEREFORE if genes are on different chromosomes they are not linked - Mendel’s ratios (9:3:3:1) do not apply to linked genes

Question 18

How and where in meiosis can linked genes be affected?

Answer 18

• in PROPHASE I of meiosis homologous chromosomes form bivalents where crossing over occurs, this can produce RECOMBINANTS in linked genes
• if linked genes on the chromosome are very CLOSE together it is unlikely crossing over will separate them and so inherited together HOWEVER if linked genes are FAR apart there is a chance of separation by crossing over - INCOMPLETE LINKAGE, this is rare

Question 19

What are mutations? rates? consequences? occurs?

Answer 19

• MUTATIONS are spontaneous random events, causing an unpredictable change in the genetic material of an organism
• mutation rates are normally very LOW, but in organisms with short life cycles and more frequent cell division, the rate of mutation is HIGHER
• it is the source of genetic VARIATION which can result in evolution through natural selection
• most mutations occur during CROSSING OVER in prophase-I and non-disjunction (failure of homologous chromosomes to separate) in anaphase-I and anaphase-II.

Question 20

Gene point mutations VS chromosomal mutations? examples?

Answer 20

• Gene (point) mutations - affect SINGLE bases in a gene (i.e A, C, G, T; change in base sequence, change in codons then sequence of amino acids, diff aa translated, may not function) e.g sickle cell anaemia
• Chromosomal mutations - affect MANY genes, structure and number of whole chromosomes may change e.g down’s syndrome

Question 21

What external factors can increase mutation rates? examples?

Answer 21

• the rate of mutation may be increased by MUTAGENS;

• IONISING radiations (gamma radiation, UV and X-rays)
• certain chemicals (POLYCYCLIC HYDROCARBONS in cigarette smoke)

Question 22

What is a mutagen which causes cancer called? how can healthy genes form a cancer?

Answer 22

• a mutagen which causes cancer is called a CARCINOGEN - some genes called PROTO - oncogenes (healthy genes involved in cell growth/processes) can mutate to become oncogenes
• ONCOGENES are involved causing uncontrolled cell division forming a cancer

Question 23

What is sickle cell anaemia? type of genetic? type of mutation?

Answer 23

• example of GENE (point) mutation, the mutant allele is CODOMINANT (having an affected and unaffected allele means some RBCs will be sickle shaped the rest healthy)
• a SUBSTITUTION gene mutation (base pair is replaced by another, changing one amino acid in the polypeptide chain) found in the haemoglobin (Hb) gene
• abnormal Hb caused RBCs to become sickle shaped thus affecting ability of Hb to bind to oxygen

*results in anaemia, can be fatal

Question 24

What are the types of chromosome mutations? x3

Answer 24

1. STRUCTURE - errors occur when chromosomes exchange sections of DNA during crossing over at prophase I
2. WHOLE SETS - if a gamete receives two sets of chromosomes during a failed meiotic division, the gametes will be diploid rather than haploid, this is known as POLYPLOIDY (common in flowering plants i.e tomatoes and wheat, animals would not survive)
3. NUMBER (NON-DISJUNCTION) - non disjunction is a process in which a faulty cell division, homologous chromosomes do not separate, resulting in one daughter cell getting two copies of a chromosome, whilst the other gets none