BIO 156

Chapter 17

Principles of Human Heredity

Meiosis and Gamete Formation

• During gamete formation, germ cells undergo a special kind of division called meiosis, which reduces the number of chromosomes by half.– Meiosis is a type of nuclear division found only in germ-cell

production in the gonads of sexually reproducing animals.

• Meiosis involves two cellular divisions. • Meiosis I is a reduction division.

– The chromosome number is halved.• Meiosis II is like mitosis in many ways.

– When haploid cells divide in meiosis II, they produce two new cells.

– Each cell contains a haploid number of single-stranded chromosomes.

Meiosis I

Meiosis II

Meiosis in Sperm Formation

Meiosis in Ovum Formation

Principles of Heredity

• Much of our early knowledge on heredity came from the work of Gregor Mendel.

• Mendel discovered that the traits he was studying did not blend.

• Mendel discovered that the parents contributed equally to the characteristics of their offspring.

• Mendel also discovered the principle of dominance. – A dominant factor masks a recessive factor. – A recessive factor is expressed only when the

dominant factor is missing. – The dominant and recessive genes are

alternative forms of the gene, or alleles.– Three genetic combinations are possible for a

given trait: • Heterozygous

• Homozygous dominant

• Homozygous recessive

• The genotype of an organism is its genetic makeup; the phenotype is its appearance.

• Genotypes and phenotypes can be determined by the Punnett square.

Monohybrid Cross

• Genes on different chromosomes segregate independently of one another during gamete formation. – This is the principle of

independent assortment. It holds true only for nonlinked genes.

Mendelian Genetics in Humans • Autosomal-recessive traits are expressed only

when both alleles are recessive. – Human cells contain 23 pairs of chromosomes:

• 22 pairs of autosomes and 1 pair of sex chromosomes.

– Chromosomes carry dominant and recessive traits.

– Inheritance of these traits is consistent with Mendel’s principles of inheritance.

• Autosomal-dominant traits are expressed in all individuals.

Variations in Mendelian Genetics

• Incomplete dominance results in a kind of blending of traits, called incomplete, or partial, dominance.

• Some genes have multiple alleles. – Multiple alleles result in more genotypes

and phenotypes in a population. – Because chromosomes exist in pairs,

individuals can have only two of the possible alleles.

– Codominance occurs in multiple-allele genes. Codominant genes are expressed fully and equally.

• Some traits are determined by more than one gene pair. – This phenomenon is referred to as

polygenic inheritance.

End of Chapter 17