Ocr A level Biology: 6.2 Patterns Of Inheritance

Genotype is:

the genetic makeup of an organism
(the alleles present in an organism)

Phenotype is:

The physical characteristics of an organism
(e.g. tall, brown eyes, red petals, AB blood group)

The phenotypic variation of an organism is influenced by:

Both its genotype and its environment

Genetic factors that cause variation in phenotype are:

Mutations to DNA or chromosomes,
and sexual reproduction

Mutagens are:

Agents that cause genetic mutations
(e.g. UV light, colchicine, X-rays and transposons)

Mutations may be advantageous, harmful, or:

Neutral

Chromosome mutations occur during which part of the cell cycle?

Meiosis

A chromosome mutation that results in part of the chromosome being lost, including genes and regulatory sequences, is:

Deletion

A chromosome mutation that results in a section of chromosome breaking off, turning through 180 degrees, and re-joining the same chromosome is

Inversion

A chromosome mutation that results in one pair of chromosomes or chromatids failing to separate, and results in gametes with less, or more chromosomes than the haploid number is:

Non-disjunction

Heterozygous is:

Where there are different alleles at a particular locus on a pair of homologous chromosomes
(e.g. Tt)

An allele is:

An alternative form of a gene (e.g. T or t)

Homozygous

two identical alleles for a characteristic

Monogenic is:

Where a character (also known as a trait) is controlled by a single gene
(e.g. height in pea plants)

Polygenic is:

Where a character (also known as a trait) is controlled by many genes
(e.g. height in humans)

A monohybrid cross is:

A cross involving only one gene, that has two contrasting physical traits
(e.g. height: tall or dwarf)

A Punnett square is:

A grid that is used to work out
genotypes and phenotypes

steps for performing a genetic cross

parents phenotype, parents genotype, gametes of each parent and circle the letters, punnet square, proportion of each genotype in offpsring %, proportion of each phenotype in offspring %

A dihybrid cross is:

A cross involving two genes, each on separate homologous chromosomes. Each gene has two contrasting physical traits (e.g. height: tall or dwarf, and flower colour: white or red)

The F1 generation is:

The first filial generation. These are the offspring of the cross between two parents.

The F2 generation is:

The second filial generation. These are the offspring of two F1 individuals

Mendel's first pea plant monohybrid cross

homozygous dominant green pea plant and homozygous recessive yellow pea plant

Mendel's pea plant F1 generation

100% heterozygous pea plants, all with green pods

Mendel's second pea plant monohybrid cross

heterzygous green pea plant and heterozygous green pea plant

Mendel's pea plant F2 generation

3 green pod pea plants: 1 yellow pod green plant

Ratio for a monohybrid cross between 2 heterozygous parents

3 (dominant): 1(recessive)

codominance

two different alleles occur for a gene - both of which are equally dominant, as a result both alleles of the gene are expressed in the phenotype of the organism if present

example of codominance

snapdragon flowers

resulting phenotypes in snapdragon flowers

red flowers - if plant is homozygous for allele that codes for production of red pigment
white flowers - if plant is homozygous for allele that codes for no pigment production
pink flowers - if plant is heterozygous, single allele which codes for red pigmentation produces enough pigment to produce pink flowers

how to write codominant alleles using snapdragon example

upper and lower cases not used, instead a letter is chosen e.g. C for colour of flowers, different alleles are then represented with another letter e.g. CR (imagine the R as superscript) for the allele coding for red flowers and CW (imagine the W as superscript) for the allele coding for white flowers

The characteristic for which there are three or more alleles in a population's gene pool, is known as:

Multiple alleles

example of multiple alleles

blood group

blood group

determined by gene with multiple alleles, immunoglobulin gene (Gene I) codes for production of antigens present on the surface of red blood cells

different blood group alleles

IA = antigen A
IB = anitgen B
IO = no antigen

codominant blood group alleles

IA and IB

recessive blood group alleles

IO

blood group combinations

IAIA/IAIO = blood group A
IBIB/IBIO = blood group B
IAIB = blood group AB
IOIO = blood group O

multiple alleles cross for blood group

a cross between IOIA and IOIB can produce any of the four blood groups

The term 'sex-linked' refers to:

A gene that is present on (one of) the
sex chromosomes

Sex chromosomes have non-homologous regions. This means:

The shorter Y chromosome lacks some
of the gene loci of the X chromosome
(only part of the X and Y chromosomes
contain the same genes at the same loci).

First example of sex-linked gene

colour blindness

colour blindness

caused by a recessive allele on the X chromosome, it is therefore much more likely in males (to get colour blindness in female offspring father has to be colour blind whereas neither of the parents has to be colourblind for males to get colour blind)

Second example of sex-linked gene

Haemophilia

Haemophilia

sex-linked genetic disorder, unable to clot blood fast enough due to absence of protein blood-clotting factor, coded for by a recessive allele

alleles for haemophila and normal alleles

XH = dominant healthy allele
Xh = recessive allele coding for haemophilia
Y = y chromosome

haemophilia cross between a normal female carrier and a normal male (XHXh and XHY)

25% normal female (XHXH), 25% normal male (XHY), 25% normal female carrier (XHXh), 25% haemophiliac male (XhY)

haemophilia cross between a normal female and haemophiliac male (XHXH and XhY)

50% carrier females (XHXh), 50% normal males (XHY)

Assuming there is no crossing over, the ratio of phenotypes in a dihybrid cross between two heterozygous individuals for both genes is:

9 : 3 : 3 : 1

A gene locus is:

The position of a gene on a chromosome / pair of homologous chromosomes

A population's gene pool is:

All the alleles and genes within a breeding population

Autosomes are:

The pairs homologous chromosomes
that are not the sex chromosomes
(humans have 22 pairs of autosomes)

Autosomes are fully homologous.
This means:

They match in length and contain the same genes at the same loci

A male mammal cannot be described as heterozygous or homozygous for sex-linked genes such as haemophilia. Instead they are described as:

Functionally haploid, or hemizygous

XX is described as the homogametic sex. XY is described as the heterogametic sex. In birds and butterflies, which gender is the homogametic sex, and which the heterogametic sex?

Homogametic = males
Heterogametic = females

XX is described as the homogametic sex. XY is described as the heterogametic sex. In mammals, which gender is the homogametic sex, and which the heterogametic sex?

Homogametic = females
Heterogametic = males

Sickle cell anaemia is caused by a mutation in the gene that codes for the β-globin chain of haemoglobin. Exactly where, in the red blood cell, is the β-globin chain synthesised?

Rough endoplasmic reticulum
(ribosomes)

With reference to sickle cell anaemia; a sufferer is someone with both sickle cell alleles. Heterozygotes for this condition are termed:

Symptomless carriers

Autosomal linkage is:

Two or more gene loci are present on the same autosome (non-sex chromosome).
These gene loci are often inherited together

In eukaryotic organisms, the number of linkage groups is equal to the number of pairs of autosomes in that species. Tigers (Panthera tigris) have 38 chromosomes. How many linkage groups do they have?

36

In eukaryotic organisms, the number of linkage groups is equal to the number of pairs of autosomes in that species. Wolves (Canis lupus) have 78 chromosomes. How many linkage groups do they have?

76

Phenotype ratios for dihybrid inheritance from two heterozygous parents, where the two gene loci are on separate pairs of homologous chromosomes, is 9:3:3:1 What are the phenotype ratios when the genes are linked?

3 (dominant phenotype) : 1 (recessive phenotype)

In tomato plants, the genes for leaf colour and fruit texture are linked (green leaf (G); mottled (g): smooth fruit (S); peach fruit (s)). What are the offspring genotypes that two heterozygote parents produce? Assume no crossing over.

GGSS, GgSs, GgSs, ggss

In tomato plants, the genes for leaf colour and fruit texture are linked (green leaf (G); mottled (g): smooth fruit (S); peach fruit (s)). What are the offspring genotypes when a heterozygote undergoes a test cross? Assume no crossing over

GgSs, ggss

In tomato plants, the genes for leaf colour and fruit texture are linked (green leaf (G); mottled (g): smooth fruit (S); peach fruit (s)). What are the offspring genotypes when a heterozygote undergoes a test cross? Assume crossing over occurs.

GgSs, ggss, GGss, ggSs

In tomato plants, the genes for leaf colour and fruit texture are linked (green leaf (G); mottled (g): smooth fruit (S); peach fruit (s)). What are the offspring phenotypes that two heterozygote parents produce? Assume no crossing over.

3 green/smooth : 1 mottled/peach

Crossing over is where:

Sections of non-sister chromatids detach and recombine. This produces gametes with different allele combinations in gametes that were present in the parent.

Crossing over occurs during which stage of meiosis?

Prophase 1

Epistasis is:

The interaction of non-linked gene loci, where one gene masks the expression of the other

The gene locus that affects the second gene locus, is known as the:

Epistatic gene

The gene locus that is affected by the epistatic gene, is known as the:

Hypostatic gene

Epistasis works in two ways. These are:

1/ Antagonistic
(recessive epistasis and dominant epistasis)
2/ Complementary (each gene locus codes for an enzyme, each needed in a metabolic pathway)

Salvia flower colour (recessive epistasis), is controlled by two gene loci on different chromosomes. 'aa' produces white flowers regardless of the hypostatic gene. The phenotype ratio of offspring from a cross between AaBb x AaBB are:

6 purple : 2 white
[3 : 1]

Recessive epistasis results in the typical phenotypic ratio, when two heterozygous individuals for both genes are crossed, of:

9 : 3 : 4

Salvia flower colour (recessive epistasis), is controlled by two gene loci on different chromosomes. 'aa' produces white flowers regardless of the hypostatic gene. The phenotypes of offspring from a cross between Aabb x AaBb are:

3 purple : 3 pink : 1 white

Feather colour in chickens (dominant epistasis), is controlled by two gene loci on different chromosomes. The 'I' of the epistatic gene prevents the formation of colour, regardless of the hypostatic gene. The phenotypes of a cross between
Iicc x IiCc are:

6 white : 3 coloured
[2 : 1]

Dominant epistasis results in the typical phenotypic ratio, when two heterozygous individuals for both genes are crossed, of:

13 : 1
or
12 : 3 : 1

Feather colour in chickens (dominant epistasis), is controlled by two gene loci on different chromosomes. The 'I' of the epistatic gene prevents the formation of colour, regardless of the hypostatic gene. The phenotypes of a cross between
Iicc x Iicc are:

All white

Complementary epistasis results in the typical phenotypic ratio, when two heterozygous individuals for both genes are crossed, of:

9 : 3 : 4 or
9 : 7 or
9 : 3 : 3 : 1

ppRR and ppRr genotypes produce which comb shape phenotype in domestic chickens?

Rose

The chi squared test is a statistical test designed to:

Find out if the difference between the observed and expected data is significant (i.e. because of true biological rules) or due to chance

The null hypothesis states:

"There is no statistically significant difference between the observed and expected data. Any difference is due to chance"

The chi squared test can be used when:

Data are in categories
Sample size is large
Data are raw counts
There are no zero scores in the raw data

In the chi squared test, the null hypothesis is accepted when:

The chi squared value you have calculated is smaller than the 0.05 (5%) p value given in the table, at the correct degrees of freedom

It was hypothesised that equal numbers of aphids would be found on 5 different tree species. How many degrees of freedom is appropriate for the chi squared test?

4
(n - 1)

It was hypothesised that equal numbers of aphids would be found on 5 different tree species. You calculate a chi squared value of 4.66
The p = 0.05 value in the table is 9.49. Accept or reject the null?

Accept the null
[Any difference between the observed and expected data is only due to chance,
and is not significant]

It was hypothesised that equal numbers of aphids would be found on 5 different tree species. You calculate a chi squared value of 4.66 The p =0.05 value in the table is 9.49 What is your conclusion?

The calculated value of chi squared is smaller than the p = 0.05 probability value at 4 d.f. so the
null hypothesis is accepted. The difference between the observed and expected data is not significantly different. Aphids have no preference of tree species

A cross between two mice, each heterozygous for coat colour [Yy], was performed. What is the expected ratio of offspring phenotypes? Y = yellow, y = white

3 yellow: 1 white
[typical monohybrid ratio]

A cross between two mice, both heterozygous [Yy] for coat colour, was performed. A chi squared value of 16.77 is calculated. The p = 0.05 value in the table is 3.84 Is the null hypothesis accepted or rejected?

Reject the null
[The difference between the observed and expected data is not due to chance.
The difference is significantly different]

A cross between two mice, each [Yy] for coat colour, was performed. A chi squared value of 16.77 is calculated.
The 5% p value in the table is 3.84
A student concludes a typical 3:1 monohybrid ratio has been observed. Does the data support her conclusion?

No, the student's conclusion is not supported. The null is rejected as the observed ratio is statistically different from the expected ratio as the chi squared number is greater than the test statistic.

It was hypothesised that equal numbers of aphids would be found on 5 different tree species. You calculate a chi squared value of 4.66 The 5% p value in the table is 9.49
A student concludes there is no preference of aphids to species of tree. Is her conclusion supported?

Yes, the student's conclusion is supported [the null is accepted]

An easy way to remember what to do with chi squared is:

If χ2 < p value in table >> accept null
If χ2 > p value in table >> reject null

Variation

Any difference between individuals of the same species

Causes of variation

mutation, meiosis, sexual reproduction

Chlorosis

a condition where leaves in plants look pale of yellow

Cause of chlorosis

cells are not producing the normal amount of chlorophyll

Effect of chlorosis

plant's ability to make food by photosynthesis is reduced

why do cells stop working and cause chlorosis?

environmental causes e.g. lack of light, mineral deficiencies and virus infections

Continuous variation is:

a characteristic that can take any value within a range
(e.g. human height)

Cause of continuous variation

genetic and environmental

Cause of discontinuous variation

mostly genetic