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
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