Heredity
Mitosis:
 A cell divides to produce ______ daughter cells, each having the ________ number of chromosomes as
the parent cell.

Daughter cells have the same hereditary materials or genes as the parent cell.
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Young or offspring produced asexually will therefore be ___________ ____________ to the parents and
are called __________.
Meiosis:

A type of cell division that results in the nucleus of each daughter cell containing __________ the number
of chromosomes as the nucleus of the parent cell.

Cell divides by meiosis, the daughter nucleus will contain _______ the chromosome number, i.e. each
daughter nucleus will have ______ chromosome from each pair.

This chromosome number in the gamete is the _________ number.
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Twice the haploid number of chromosomes make up the _________ number.

In Man, the diploid number is _________.

Sperm/egg has __________ number of ______ chromosomes.
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A gamete is a reproductive cell containing the haploid number of chromosomes.
Gene
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Chromosome is a thread-like structure found in the nucleus on which the genetic material is organized.
Alleles are different forms of the same gene. They occupy the same relative positions in a pair of homologous
chromosomes.
Gene locus is the place on the chromosome where gene resides.
Monohybrid Inheritance
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Homologous chromosome
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Heterozygous _____________________________________________________________________________
Homozygous ______________________________________________________________________________
Phenotype ________________________________________________________________________________
Genotype _________________________________________________________________________________
Dominant _________________________________________________________________________________
Recessive ________________________________________________________________________________
Mendel’s Law of Segregation ________________________________________________________________
__________________________________________________________________________________________
Worked example 1 from J81 1A 6 – Homozygous dominant x Homozygous recessive
In a plant-breeding experiment, red-flowered plants (RR) were crossed with white-flowered plants (rr). Give a
full genetic diagram to explain your answer.
♂
X
Parental genotypes
Red
Gametes
F1 geneotypes
F1 phenotypic ratio
♀
White
Worked example 2 from J81 1A 6 – Heterozygous x Homozygous recessive
In a plant-breeding experiment, red-flowered plants (Rr) were crossed with white-flowered plants (rr). Give a
full genetic diagram to explain your answer.
♂
♀
X
Parental genotypes
Red
White
Gametes
F1 geneotypes
F1 phenotypic ratio
Incomplete dominance
Both alleles exert their effects so that the hybrid has a phenotype that is intermediate between that found in its
parents.
Co-dominance
Both alleles have an effect on the phenotype of an individual:
Alleles:
Both IA and IB are dominant over IO, but are codominant to one another.
Genotype
Phenotype
Blood group A
Blood group A
Blood group B
Blood group B
Blood group O
Blood group AB
Worked Example 3 based on D90 2A 4 – Codominance in ABO blood group
Complete the genetic diagram below to show the possible blood groups amongst the offspring of a father who
is heterozygous for blood group A and a mother who is heterozygous for blood group B.
Father
Mother
Phenotypes
Blood group A
Blood group B
Genotypes
_______________
_______________
Gametes
_______________
_______________
_______________
_______________
Genotypes of
offspring
_______________
_______________
_______________
_______________
Phenotypes of
offspring
_______________
_______________
_______________
_______________
Inheritance of sex
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Whether a child is born male or female is determined at the moment of fertilization.
Of the 23 pairs of chromosomes in a human nucleus, one pair is known as the sex chromosomes. In the
female, the sex chromosomes are identical and are called ‘X’ chromosomes.
In the male, they are not identical. One of them is an ‘X’ chromosome, exactly like those in the female, but
the other is a shorter ‘Y’ chromosome.
Worked example 4 from D77 1A 4 and D78 1A 4
(a) Using the symbols R for red flower and r for the white flower, complete the diagram to show what would
happen if the Rr x Rr.
♂
♀
X
Parental genotypes
Red
White
Gametes
F1 geneotypes
F1 phenotypic ratio
(b) A man whose blood group is AB is married to a woman of blood group O. Their genotypes are AB and OO
respectively. Complete the diagram to show the results of the cross.
♂
♀
X
Parental genotypes
AB
OO
Gametes
F1 geneotypes
F1 phenotypic ratio
(c) Which blood group is the result of co-dominance? _____________________________________________
Variation
Sexual reproduction leads to variation in the offspring, that is, each individual has different characteristics. No
two offspring from the same parents, produced by sexual reproduction, are genetically identical. An exception
occurs when the offspring develop from the same ovum and sperm, in which case they are ‘identical twins’.
Continuous variation is the result of the interaction of two factors:
(i)
(ii)
the genes that are inherited by an individual
the effect of the environment on the individual
The environmental factors involved might include:
(i) the availability and type of food (in animals)
(ii) disease
(iii) the climate - amount of sunlight, temperature, water availability
(iv) the ions present in the soil (in plants)
(v) competition from other organisms in the environment.
In continuous variation, individual show a range between the two extremes.
Every possible form between the two extremes will exist.
Examples of continuous variation are:
(i) body mass
(ii) height
(iii) foot size
Discontinuous variation is the result of
inheritance only. There are few types,
with no intermediates.
Brought about by one or a few genes.
Worked example 5 from J97 2A 5
Seventy seeds were collected from a cross between two plants of the same species. The seeds were sown at
the same time and, after three weeks, the heights of the plants which grew were measured and found to fall
into two groups, A and B, as shown below.
(a) (i) Name the type of variation shown in:
Group A:
Group B:
(ii) State three factors which might have caused this variation.
1. ______________________________________________________________________________________
2. ______________________________________________________________________________________
3. ______________________________________________________________________________________