Sexual Reproduction and the Diversity of Life

BC Science Probe 9Section 3.2Pages 79-82

Fertilization Produces Diploid Cells

• Fertilization: The joining or fusing of two gametes.– Gametes are produced by meiosis– They are haploid (only have ½ the regular number

of chromosomes of the parent cell)– When two gametes fuse during fertilization, they

make a diploid cell (a cell with a full set of chromosomes)

Fertilization Produces Diploid Cells

• The first diploid (2n) cell of the offspring is called the zygote.

sperm(n) + egg (n) = zygote (2n)

Fertilization Produces Diploid Cells

• Zygote:– Gets half of its chromosomes from each parent.– Human gametes each have 23 chromosomes, so a

human zygote will have 46, then divide by mitosis to produce more genetically identical cells with 46 chromosomes

Diversity Among Offspring

• Diversity = variation or differences

• There is diversity among different species and also within a species.

• Sexual reproduction creates diversity

Diversity Among Offspring

• It’s the homologous pairs of chromosomes that produce the variations among members of a species.– Each of the chromosomes in the homologous pair

have corresponding genes (genes that code for the same trait ex. hairline)

– But… these genes can be slightly different from one another!

Diversity Among Offspring

• The different forms of the same gene are called alleles.– Back to the hairline example: there are two

different alleles.• One is for a straight hairline• One is for a widow’s peak

Diversity Among Offspring

• The allele for a widow’s peak is dominant.– This means that it will express its trait (show up) if

it is present

Diversity Among Offspring

• The allele for a straight hairline is recessive.– This means that it will only show up if both

chromosomes in the homologous pair have the recessive allele.

Diversity Among Offspring

• If an individual has one of each allele, the dominant one gets expressed.

Diversity Among Offspring

• Some alleles are not dominant or recessive.• There are such things as incomplete

dominance and codominance.

Diversity Among Offspring

• Incomplete dominance:– Two different alleles produce a mixture of the

traits.– Red carnation + white carnation = pink carnation

Diversity Among Offspring

• Codominance is when both traits get expressed.– This horse has one allele for white hair and one for

red. Both get expressed.

Diversity Among Offspring

• There are even some traits that are controlled by more than one gene!

• Human eye colour is one of these traits. That’s why there are so many variations!