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Female zygote Male zygote

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• Understand all the Key words.

• Describe how traits are produced in offspring.

• Briefly describe the origins of inheritance.

KEY WORDSHeredity Genetics Trait

Genotype Phenotype Gene

Dominant Recessive Allele

DNA

Trait – a specific physical characteristic.(blue eyes, brown hair, long fingers…)

Heredity – the passing on of traits.Genetics – field of science dealing with heredity.

Gregor Mendel (1822-1884)• First scientist to study heredity.• An Austrian monk.• Performed experiments by cross-breading pea plants showing various traits.

(pod colour, height, flower colour, pod shape...)

Mendel came up with the following hypotheses:

• Genes are passed from parent to offspring.

• Specific genes determine specific traits.

• Individuals carry two genes for each trait. (1 from mother's egg – 1 from father's sperm)

• Two genes split up (segregate) and each one ends up in a separate gamete.

x y

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Chromosomes are made of DNA in the form of a double helix (twisted ladder).

46 human chromosomes (genome) contain about 30 000 genes.

Gene - piece of DNA with instructions for a specific trait – hair colour, height, eye colour…

A single gene is responsible for a single trait.

– paired chromosomes with genes for the same trait arranged in the same order - one from mother, one from father.

Homologous chromosomes

gene – blue eyes gene – brown eyes

gene – black hair

Allele – alternate forms of a gene. Alleles occupy the same location on homologous chromosomes.

Two different alleles for the gene that determines hair colour.

Two kinds of alleles – dominant and recessive.

When a recessive allele combines with a dominant allele, the dominant allele determine how the person will look.

Recessive alleles only show in the offspring when two recessive genes are present.

Brown allele is dominant for eye colour.

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• Chromosomes are made of DNA.• Portions of DNA that code for a trait are genes.

• Homologous chromosomes have alleles of the same genes.• Alleles are dominant or recessive.