Mendelian Genetics
K. Sathasivan
• Basic genetic terms.
• Mendel's laws of inheritance.
• Complexities of genetic expression.
• Mendelian genetics in Humans
Basic Terms
DNA
Genes/
alleles
Locus
Genotype
Chromosome
G-A-T-C
C-T-A-G
AB,ab, Ab
mRNA
rRNA/ tRNA
Recombination,
Segregation and
independent
assortment Meiosis
Protein
Gametes
Random
fertilization
Mitosis
Diploid
cells/tissue/
organism
Phenotype
(brown eye,
pink flower,
sickle cell)
Figure 14.0 Painting of Mendel
Figure 14.0x Mendel
Figure 14.x1 Sweet pea flowers
Law of Segregation
• Allele pairs segregate (separate) during gamete
formation and the paired condition is restored by
the random fusion of gametes at fertilization.
Figure 14.1 A genetic cross
Figure 14.2 Mendel tracked heritable characters for three generations
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Figure 14.x2 Round and wrinkled peas
Figure 14.4 Mendel’s law of segregation (Layer 1)
Figure 14.4 Mendel’s law of segregation (Layer 2)
Figure 14.5 Genotype versus phenotype
Figure 14.6 A testcross
Law of Independent Assortment
• Law: The segregation of each allele pair is
independent of other allele pairs. The individual
allele pairs need to be located on separate loci, far
from each other to allow independent assortment
during gamete formation.
Figure 14.3 Alleles, alternative versions of a gene
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Figure 14.8 Segregation of alleles and fertilization as chance events
Complexities of Gene Expression
• Incomplete Dominance
• Co-dominance
• Epistasis
• Polygenic characters
• Pleiotropy
• Environmental effect
• Developmental effect
Figure 14.9 Incomplete dominance in snapdragon color
Figure 14.9x Incomplete dominance in carnations
Figure 14.10 Multiple alleles for the ABO blood groups
Figure 14.10x ABO blood types
Figure 14.11 An example of epistasis
Figure 14.12 A simplified model for polygenic inheritance of skin color
Figure 14.13 The effect of environment of phenotype
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Mendelian Inheritance in Humans
• Recessive Traits
• Albinism, Cystic Fibrosis, Tay Sachs,
• Sickle Cell, PKU, Galactosemia
• Dominant Traits
• Achondroplasia
• Alzheimer’s, Huntington, Hypercholesterolemia
Polygenic Traits
• Heart Disease
• Cancer
• Diabetes
• Alcoholism
• Schizophrenia
• Manic Depression
Figure 14.14 Pedigree analysis
Dominant Traits
Widow’s Peak, Freckles and Free Ear Lobe
Figure 14.16 Large families provide excellent case studies of human genetics
Early Detection of Human Genetic Disorders
• Amniocentesis
• Chorianic Villus Sampling (CVS)
• Ultrasound
• Fetoscopy
• Screening of embryos
Figure 14.17 Testing a fetus for genetic disorders
Summary
• Basic genetic terms.
Genotype, phenotype, gene, allele, dominant, recessive, homozygous and
heterozygous etc.
• Mendel's laws of inheritance.
Law of segregation and law of independent assortment.
• Complexities of genetic expression.
Incomplete dominance, codominance, epistasis, pleiotropy etc.
• Mendelian genetics in Humans
Dominant and recessive disorders and early detection techniques