Mendel's Genetics: Genes, Alleles, and Inheritance

Mendel and Genes

• Genes and alleles are fundamental concepts in genetics
• Alleles are variations of genes that determine traits
• Homologous chromosomes carry genes at specific loci
• The allele for purple flowers is located on one chromosome
• The allele for white flowers is located on its homologous pair

Terminology

• Locus or loci refers to the specific location of a gene on a chromosome
• Alleles determine traits
• Dominant alleles mask the expression of recessive alleles
• Homozygous individuals have two identical alleles for a gene
• Heterozygous individuals have two different alleles for a gene
• Genotype is the genetic makeup of an organism
• Phenotype is the observable characteristics of an organism

Ideas Regarding Inheritance Before Mendel

• Prior to Mendel, inheritance was thought to involve the inheritance of acquired characteristics
• Another idea was blending inheritance, where traits mixed in offspring

Mendel's Insights

• Gregor Mendel (1822-1884) discovered particulate inheritance
• He also discovered dominance/recessivity
• These insights are foundational in genetics

How to Breed Peas

• To breed peas, one must remove stamens from a purple flower
• Pollen from stamens of a white flower must be transferred to the carpel of the purple flower
• The pollinated carpel then matures into a pod
• Plant seeds from the pod
• Examine the offspring to observe their flower color.

Parental, First Filial, and Second Filial Generations

• The parental generation (P) consists of true-breeding parents
• Crossing purple and white flowers yields the first filial generation (F1)
• The F1 generation consists of hybrids, produces all plants with purple flowers
• Allowing the F1 generation to self-pollinate yields the second filial generation (F2)
• The F2 generation has a ratio of 3:1
• 705 plants had purple flowers
• 224 plants had white flowers

Law of Segregation

• The law of segregation states alleles of a locus separate during gamete formation
• Homologous chromosomes line up and separate during Meiosis I

P, F1, and F2 Generations in Terms of Alleles

• For the P generation, purple flowers have the genotype PP, and white flowers have the genotype pp
• Gametes produced by the purple flower plants carry the P allele
• Gametes produced by the white flower plants carry the p allele
• The F1 generation has the genotype Pp, resulting in purple flowers
• Gametes produced by the F1 generation carry either the P or p allele
• The F2 generation shows the following genotypes: PP, Pp, and pp
• There is an approximate ratio of 3:1 for purple to white flowers

Law of Independent Assortment

• The law of independent assortment states alleles of one gene segregate independently of those of other genes
• Non-homologous chromosomes assort independently during Meiosis I

Dependent vs. Independent Assortment

• In dependent assortment, experimental results contradict the hypothesis
• In independent assortment, experimental results support the hypothesis
• P generation: YYRR x yyrr
• Gametes: YR x yr
• F1 generation: YyRr
• Ova/Sperm: YR, Yr, yR, yr

Mendel's Publication

• Mendel published his observations in 1866
• His work was misunderstood or ignored at the time

Testcross

• In a testcross, the genotype of an unknown dominant phenotype is revealed by crossing it with a recessive phenotype individual
• If all offspring are purple, then the unknown genotype is PP
• If half of the offspring are purple and half are white, then the unknown genotype is Pp

Probability

• Probability: chance has no memory
• Probabilities of independent events are multiplied
• Separate probabilities for the same event are added

Flower Position

• If axial flower position (A) is dominant to terminal (a), and a self-fertilizing plant is heterozygous (Aa)
• 75% of the offspring will have axial flowers
• 150 out of 300 offspring will be heterozygous

Axial Flowers and Dwarf Stems

• Parental genotype = AaTt
• The gametes are AT, At, aT, at
• Desired genotypes are AAtt, Aatt, and aAtt
• Total probability = 3/16

Self-fertilizing Plant Traits

• A self-fertilizing plant is considered heterozygous for all 3 traits
• The traits are: flower position, stem length, and seed shape
• Each parent produces the following gametes: ATR, ATr, AtR, Atr, aTR, aTr, atR, atr
• Offspring will be homozygous dominant for flower position
• Offspring must be homozygous recessive for stem length with heterozygous seed shape
• Total probability = 1/32

Incomplete Dominance

• Incomplete dominance results in a blending of traits
• Red flowers (CRCR) crossed with white flowers (CWCW) results in pink flowers (CRCW) in the F1 generation

Co-dominance

• In co-dominance, both alleles are expressed
• This is evident in the determination of ABO blood groups by multiple alleles

Epistasis

• Epistasis occurs when alleles at one locus alter the expression of alleles at another locus in the chromosome

Polygenic Inheritance

• Polygenic inheritance involves multiple genes affecting a single trait

Environmental Influence

• Reaction norms and environmental influence affect phenotype

Pedigrees

• Pedigrees are diagrams that show the inheritance of traits across generations

Dominant or Recessive Traits

• Alkaptonuria is a recessive trait

Pleiotropy

• Pleiotropy occurs when one gene influences more than one characteristic

Amniocentesis and Chorionic Villus Sampling

• Amniocentesis and chorionic villus sampling (CVS) are prenatal diagnostic techniques
• They are used to detect genetic and chromosomal abnormalities in the fetus.