Monohybrid inheritance involves one characteristic controlled by a gene with contrasting traits. (p.230)
Genetics is the study of heredity and variation; Gregor Mendel is known as the father of modern genetics. (p.230)
Mendel used pea plants (Pisum sativum) because they grow easily, have male and female structures, have a short generation interval and clear contrasting traits. (p.231)
In Mendel's monohybrid cross, a purebred tall (TT) plant crossed with a purebred dwarf (tt) plant gave an F1 generation that was all tall (Tt). (p.231)
Selfing F1 (Tt x Tt) gave an F2 generation with a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 tall : 1 dwarf. (p.231)
Dominance principle: a dominant factor suppresses a recessive factor when both are present. (p.231)
A gene is the basic unit of inheritance, a DNA segment at a specific locus on a chromosome. (p.232)
An allele is an alternative form of a gene at the same locus on a homologous chromosome pair. (p.232)
A characteristic is a heritable feature such as height; a trait is a variation of that characteristic such as tall or dwarf. (p.232)
Phenotype is the observable characteristic; genotype is the genetic composition. (p.232)
A dominant allele always shows its trait when present and is written as a capital letter; a recessive allele shows its trait only when both alleles are recessive and is written as a small letter. (p.233)
Purebred individuals carry two identical alleles; hybrids are offspring of two purebred varieties. (p.233)
Homozygotes have two identical alleles; heterozygotes have two different alleles. (p.233)
The parental generation is the first generation of two individuals mated; the filial generation is the successive generation from purebred parents. (p.233)
Mendel's First Law (Law of Segregation): a characteristic is controlled by a pair of alleles, and only one allele from the pair is inherited in a gamete. (p.237)
Fertilisation is random, so genotype ratios can be predicted using a Punnett square. (p.235)
11.2 Dihybrid Inheritance
Dihybrid inheritance involves two characteristics, each controlled by a different gene at a different locus. (p.238)
In a dihybrid cross of round yellow (BBKK) and constricted green (bbkk) pea seeds, the F1 are all round yellow (BbKk). (p.238)
Selfing F1 gives 16 F2 genotype combinations, with a phenotypic ratio of 9 round yellow : 3 round green : 3 constricted yellow : 1 constricted green. (p.239)
Mendel's Second Law (Law of Independent Assortment): each allele from one pair can combine randomly with any allele from another pair during gamete formation. (p.239)
11.3 Genes and Alleles
Each chromosome carries many genes; the number of genes coding for proteins in a human haploid set is estimated at 25 000. (p.240)
Locus is the specific location of a gene on a chromosome. (p.240)
On a homologous pair, genes are homozygous when alleles are the same and heterozygous when they differ. (p.240)
11.4 Inheritance in Humans
Human chromosomes are autosomes (22 pairs) and sex chromosomes (one pair). (p.242)
A human somatic cell has 44 autosomes and 2 sex chromosomes; males are XY and females XX. (p.242)
Karyotype is the number and structure of chromosomes in a cell nucleus, arranged in homologous pairs. (p.242)
Nondisjunction during meiosis causes abnormal chromosome numbers in gametes and zygotes. (p.243)
Down syndrome (trisomy 21) has 47 chromosomes with an extra chromosome 21. (p.243)
Turner syndrome has 45 chromosomes (44 + XO), is female, and has a missing X chromosome. (p.244)
Klinefelter syndrome has 47 chromosomes (44 + XXY), is male, and has an extra X chromosome with underdeveloped secondary sex characteristics. (p.244)
ABO blood group is an example of multiple alleles: IA and IB are codominant and dominant over IO. (p.244)
Blood group phenotypes and genotypes: A (IAIA or IAIO), B (IBIB or IBIO), AB (IAIB), O (IOIO). (p.245)
Rhesus factor: Rh+ is dominant and Rh- is recessive; Rh+ can be homozygous (Rh+Rh+) or heterozygous (Rh+Rh-). (p.246)
Thalassemia is an inherited disease caused by gene mutation on chromosome 11 or 16, causing low haemoglobin and small, pale red blood cells. (p.246)
A thalassemia carrier (minor) has one recessive allele and no symptoms; thalassemia major occurs when both alleles are recessive. (p.247)
Sex is determined at fertilisation: sperm carry either X or Y, and the egg always carries X. (p.247)
Sex-linked genes are located on the X chromosome, e.g. colour blindness and haemophilia. (p.247)
Colour blindness is caused by a recessive allele on the X chromosome and occurs mostly in males. (p.248)
Haemophilia is caused by a recessive allele on the X chromosome; blood cannot clot normally. (p.249)
Ability to roll tongue is dominant; free earlobe is dominant and attached earlobe is recessive. (p.249)
A family pedigree is a flowchart across generations that shows inheritance of characteristics and can reveal dominant or recessive genes. (p.250)