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Chpt. 17 Genetic Crosses. Gregor Mendal is known as the father of genetics!!!. General Definitions Genetics: is the study of inheritance. Somatic Cells: are all cells except sex cells e.g. Cheek, liver, muscle, blood, leaf, stem etc. Gametes (sex cells): are haploid cells that

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Chpt. 17 Genetic Crosses

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Chpt. 17

Genetic Crosses


General Definitions

  • Genetics: is the study of inheritance.
  • Somatic Cells:are all cells except sex cells
  • e.g. Cheek, liver, muscle, blood, leaf, stem
  • etc.
  • Gametes (sex cells): are haploid cells that
  • are capable of fusion.
  • Fertilisation:is the union of two gametes
  • to form a single called a zygote.

Genetic Crosses – Definitions

  • Alleles: are different forms of the same
  • gene.
  • Eg. Rabbits may have long ears (L) or short ears (l). So, the gene for ears has two alleles L or l.
  • Dominant: means that the allele prevents
  • the recessive allele working.

Genetic Crosses – Definitions

Examples of dominant traits:

Genes for

Brown eyes

Ear lobes

Dimples in chin


Curly hair

Tongue rolling


Genetic Crosses - Definitions

  • Recessive: means that the allele is
  • prevented from working by a dominant
  • allele.
  • Examples include:

- Cystic fibrosis

- Albinism

- Haemophilia

Eg. A rabbit with the alleles Ll has long ears because the recessive allele for short ears (l) is prevented from working by the dominant allele for long ears (L).


Genetic Crosses – Definitions

  • Genotype: means the genetic make up of an
  • organism, i.e. the genes that are present.
  • E.g. The possible genotypes for ear lengths
  • in rabbits are LL, Ll, ll

Genetic Crosses – Definitions

  • Phenotype: means the physical make-up or
  • appearance, of an organism.
  • - Eg. The phenotypes of the rabbits are either long ears or short ears.
  • - It is the interaction of the genes
  • (genotype) with the environment that produces the phenotype:
  • Phenotype = Genotype + Environment

Genetic Crosses – Definitions

  • Homzygous (Pure Breeding): means that both alleles
  • are the same. i.e. The genotypes LL and ll are both
  • homozygous.

Genetic Crosses - Definitions

  • Heterozygous (Hybrid): means that the alleles are
  • different. i.e. the genotype Ll is heterozygous.

Genetic Crosses - Definitions

  • Progeny: refers to offspring that are
  • produced:
  • F1 progeny means the first generation of
  • offspring (first filial generation).

Genetic Crosses

  • Points to note:
    • a pair of alleles is present in the cells
    • of an organism for each characteristic.
    • only one allele for each characteristic
    • is carried in each gamete.

Genetic Crosses – Questions

Question 1:

In rabbits, long ears(L) are dominant over short ears (l). Show the genotypes and phenotypes for the offspring of a cross between two rabbits whose genotypes are (LL) and (ll).

Solution 1:

Parents Genotypes LL x ll

Genotypes of Gametes L x l

F1 Genotype of Offspring Ll

F1 Phenotype of offspring All long ears


Genetic Crosses – Questions

When answering genetic questions a punnett square is used for the more difficult questions.

Punnett Square: is a grid used to show the ratio of the genotypes of the first generation of offspring (progeny) in a genetic cross.

Six sample questions will be completed involving the use of a Punnett Square (see attached leaflet)


Incomplete Dominance

  • Incomplete dominance (codominance): means that neither allele is dominant or recessive. Both alleles work in the heterozygous condition to produce an intermediate phenotype.
  • Incomplete dominance is relatively rare.
  • Example 1:
  • In shorthorn cattle:
  • - the genotype RR produces a red coat
  • - the genotype rr produces a white coat
  • - The heterozygous genotype Rr gives a roan coat ( patches of red and patches of white)

Incomplete Dominance

Example 2:

In snapdragons (flower):

- the genotype RR produces red flowers

- the genotype rr produces white flowers

- the heterozygous genotype Rr produces pink flowers


Pedigree Studies

Pedigree: is a diagram showing the genetic history of a group of related individuals.

See pg. 168Question 7


Sex Determination

  • Normal body cells in humans contain 46 chromosomes:
  • - 44 chromosomes – have genes that do not control sexuality – called autosomes.
  • - 2 chromosomes – control sexuality – heterosomes.
  • Autosomes– control features independent of whether a person is male or female:
  • - skin colour
  • - number of arms
  • - formation of saliva - production of digestive enzymes

Sex Determination

  • Heterosomes- these two chromosomes are the X and Y chromosomes.
  • - male body cells are XY
  • - female body cells are XX
  • The arrangement of XX for females and XY for males has the following two consequences:
  • - it is the male who determines the sex of the child.
  • - the ratio of male to female births should be 1:1.

Sex Determination in Other Species

The pattern of sex determination in some species is the reverse of that in humans.

For example in birds, moths and butterflies:

- male body cells – XX

- female body cells – XY

As a result of the this it is the female who determines the sex of the offspring.


Mendel’s Experiments

  • (Higher Level)
  • Gregor Mendel – ‘The Father of Genetics’
  • Research carried out around 1860 but ignored until
  • 1900’s
  • Mendel’s work resulted in two basic laws of
  • inheirtance – ‘Mendel’s 1st and 2nd laws’

Mendel’s 1st Law – Law of Segregation

  • Inherited traits are controlled by two alleles
  • The alleles separate at gamete formation, with each
  • gamete only having one allele
  • Example:
  • Consider a species of plant whose height is controlled by a pair of alleles Tt. When gametes are formed only one allele can enter each gamete i.e. the alleles segregate



Pair of gametes each having only one allele



Mendel’s 1st Law – Law of Segregation

  • Behaviour of chromosomes according to Mendel’s 1st law:
  • Diploid organisms – chromosomes occur in
  • homologous pairs.
  • Pairs of alleles occupy same position on a homologous
  • pair.
  • During meiosis, homologous chromosomes separate
  • and go into different cells.
  • As a result pairs of alleles also separate


Each cell has only one chromosome and one allele






Monohybrid Cross:involves the study of a single characteristic e.g. eye colour, seed shape etc.

Dihybrid Cross: involves the study of two characteristics at the same time e.g. studying plant size (tall or small) and pod colour (green or yellow) at the same time.


Mendel’s 2nd Law – ‘Law of Independent Assortment’

When gametes are formed .....

either of a pair of alleles ......

is equally likely .......

to combine with either of another pair of alleles .......



A can combine with either Bor b

a can combine with either B or b

Four types of gamete can form

AB Ab aB ab


Mendel’s 2nd Law – ‘Law of Independent Assortment’

  • Behaviour of chromosomes according to Mendel’s 2nd Law:
  • At gamete formation...
  • either of a pair of homologous chromosomes....
  • is equally likely..............
  • to combine with either chromosome of a second
  • homologous pair

Dihybrid Crosses

Example 1:

In pea plants, tall plant (T) is dominant over small plant (t). In addition, green pod (G) is dominant over yellow pod (g).

A tall plant with green pods (homozygous for both traits) is crossed with a small plant with yellow pods.

Why is this a dihybrid cross?

Show by diagrams the genotypes and phenotypes of the progeny of this cross.


Example 2:

In guinea pigs, black coat (B) is dominant to white coat (b). Also short hair (S) is dominant to long hair (s).

Show the genotypes and phenotypes of the F1 progeny for a cross involving a black coated, short-haired guinea pig (heterozygous for both traits) and a white-coated, long-haired animal.

State the expected ratio of the offspring


Example 3:

A homozygous purple-flowered, short stemmed plant was crossed with a red flowered, long stemmed plant.

All the F1 offspring were purple-flowered with short stems.

State the dominant and recessive traits.

Explain, using diagrams, why the F1 plants all had the same phenotypes

Give the expected ratios if an F1 plant is selfed.


Mendel’s 2nd Law – Expected Ratios

Case 1 (Dihybrid Cross):

Heterozygous X Homozygous Recessive

(both characteristics)

Ratio of offspring = 1 : 1 : 1 : 1

Independent Assortment has occurred!!!

Case 2 (Dihybrid Cross):

Heterozygous X Heterozygous

(both characteristics) (both characteristics)

Ratio of offspring = 9 : 3 : 3 : 1

Independent Assortment has occurred!!!



  • Means that both genes are present on the same
  • chromosome
  • Linked genes are passed on together

Linked Genes

R and S

r and s

No Linkage

T and t







Pair of alleles

Homologous Pair



  • Linkage contradicts Mendel’s 2nd law of independent
  • assortment (see following examples)
  • Much less variation in offspring
  • *Note: The position a genes occupies on a chromosome
  • is called the locus

Linkage Example 1:

Draw simple chromosome diagrams to illustrate the following cells. In each case show the gametes that might be produced.

a) The genes are not linked and the genotype is AaBb.

b) The genes are linked (A to B and a to b) and the genotype is AaBb.


Linkage Example 2:

Draw simple chromosome diagrams to show each of the following cells. In each case indicate the gametes that each cell might produce.

a) Genotype RrSs, genes are not linked

b) Genotype RrSs, genes are linked (R to S and r to s)

c) The genes are not linked, the cell is homozygous for R and heterozygous for S

d) The genes are linked, and the cell is homozygous dominant for both genes.


Linkage Example 3:

Show the expected genotypes of the progeny for the following crosses, AaBb x aabb:

a) where there is no linkage

b) where the genes are linked, A to B and a to b


Sex Linked (X Linked)

  • Sex chromosomes in humans:
  • X – carries a large number of genes
  • Y – much shorter than X and carries
  • very few genes
  • Sex linkage means that a characteristic is controlled by
  • a gene on an X chromosome.
  • Examples of traits controlled by a gene on the X
  • chromosome:
  • Colour blindness


Duchenne muscular dystrophy

  • In sex linked characteristics, the recessive phenotype is
  • more likely to occur in males i.e. males suffer more
  • often from sex linked characteristics.
  • Note: in males there is no corresponding allele on the Y chromosome

Sex Linked (X Linked)

  • Females tend to be carriers - they carry the gene for
  • the trait but do not suffer from the disease

Carrier Mother & a Colour Blind Father

Parents GenotypeNXXn nXY

Gametes NX nX nX Y

Offspring Genotype NXnX NXY nXnX nXY

Offspring Phenotype:

female male female male

carrier normal CB CB



  • A colour-blind female inherits the colour-blind gene
  • from her mother( a carrier) as well as from her father -
  • both parents must have the gene.
  • For a boy to be colour-blind, it is necessary only that his
  • mother is a carrier. This is far more common and the
  • reason why far more boys are colour-blind than girls


  • Haemophilia – is a bleeding disorder caused by the
  • lack of a particular blood protein
  • Caused by a gene located on the X chromosome:
  • - allele (N) for production of clotting protein is dominant.
  • - recessive allele (n) does not carry the correct genetic code for the production of the protein.
  • More common in males, extremely rare in females

Non-nuclear Inheritance

  • Extra-nuclear genes are present as small circles of
  • DNA in mitochondria and chloroplasts (both of which
  • reproduce by themselves passing on their genes)
  • Since, pollen does not contain these organelles and
  • mitochondria are in the tail of the sperm, only the
  • head joins with the egg, this means that
  • mitochondria and chloroplasts are inherited from
  • the female in the cytoplasm of the egg.
  • Mitochondrial DNA (mtDNA) in humans is inherited
  • only from the mother, subsequently, a number of rare
  • human disorders are inherited only from the mother
  • because they are controlled by extra-nuclear genes
  • located on mtDNA.

Mitochondrial Eve

Please read over paragraph relating to this topic pg. 179 in your book.