Unit 1 Mendelian Inheritance Patterns (dominant, recessive), Modified Mendelian Inheritance

Genetics – The branch of biology that deals with heredity and variation of organisms. 

Things you should know:

• Chromosomes contain our genetic information as DNA. Each chromosome consists of a single, long DNA molecule that is wrapped around proteins called histones to help pack it tightly. 
• During DNA replication, this single DNA molecule makes an exact copy of itself. As a result, each chromosome then has two identical DNA molecules, known as sister chromatids, connected at a region called the centromere. These chromatids separate during cell division (mitosis).
• DNA itself is made up of repeating units called nucleotides.
• Genes are specific sequences of nucleotides within the DNA that provide instructions for making proteins or other important molecules. Each gene is located at a specific position on the chromosome, known as a locus (plural: loci). Therefore, by definition, a gene is the functional unit of DNA which encodes a polypeptide product.
• Consider a non-replicating diploid cell. Then, chromosomes and genes occur in pairs, and are called homologous chromosomes – one from each of its two parents. This diploid set includes two copies of each chromosome. During sexual reproduction, homologous chromosomes exchange segments of DNA while forming gametes (sperm and eggs), creating new genetic combinations.  The haploid gametes fuse during fertilization to form the zygote, restoring the diploid chromosome number. 

Central Dogma 

Mendelian Theory of Inheritance

Gregor Johann Mendel:

• Known as the ‘Father of Genetics’
• Austrian Monk
• Worked with pure-line peas (Pisum sativum) – considered seven traits or characteristics of pea plants
• Published his paper “Experiment on pea hybridisation” in the Natural Science Society
• Could not repeat his work in hawk weeds as it is an asexually reproducing plant
• Work was largely ignored and eventually lost
• It was rediscovered independently by Hugo deVries, Carl Correns, and Erich von Tschermark
• Reasons for Mendel’s success:
  • He considered one or two characters at a time.
  • He always selected true breeding varieties.
  • He kept complete records of breeding experiments.
  • He selected those characters which did not show linkage or interaction or incomplete dominance.
  • He was lucky in the fact that genes for the 7 characters selected by him were mostly present on separate homologous chromosomes.

Traits studied by Mendel and their alternative phenotypes in the pea plant:

Trait	Phenotypes
Height of the plant	Tall/Dwarf
Seed colour	Yellow/Green
Seed shape	Round/Wrinkled
Pod colour	Green/Yellow
Pod shape	Inflated/Constricted
Placement of flowers	Axial/Terminal
Colour of petals	Purple/White

Some genetics key words:

• Gene: The unit of heredity; functional unit of chromosome and a segment of DNA coding for a polypeptide
• Genome: The entire set of genes in an organism
• Alleles: Alternate forms of a gene (two or more), occupying the same position on homologous chromosomes and are responsible for the same trait
• Locus: A fixed location on a strand of DNA (chromosome) where a gene (or an allele) is located
• Homozygous: Having the same alleles for a gene on both of homologous chromosomes
• Heterozygous: Having different alleles of a gene on its homologous chromosomes
• Dominant: The allele that masks/suppresses the expression of another allele for a gene is said to be the dominant allele. Dominance is seen in a heterozygous condition
• Recessive: The allele whose expression is being masked/suppressed by another allele is said to be recessive. Recessive alleles can be expressed only when present in homozygous condition, otherwise their expression is masked.
• Genotype: The genetic composition of animals with respect to a particular trait. It is the specific combination of alleles an individual has for a given gene or genes.
• Phenotype: – The physical appearance of an organism is the phenotype. It refers to the observable physical characteristics, traits, or behaviors of an organism, resulting from the interaction between its genotype (genetic makeup) and the environment.
• Monohybrid cross: A genetic cross involving a single pair of genes (one trait); two parents differ by a single trait
• P: Parental generation
• F₁: First filial generation 
• F₂: Second filial generation 
• Test Cross: Crossing an individual with a dominant phenotype but an unknown genotype with its homozygous recessive parent is a test cross
• Back cross: Crossing a F1 progeny with either of its parental genotypes is called a back cross.

Mendel’s Laws:

Mendel derived a total of three principles:

1. Law of dominance
2. Law of segregation
3. Law of independent assortment

The first two laws were derived from the monohybrid cross and the third from the dihybrid cross.

Monohybrid cross:

For stem length (height) of the pea plant:

Mating of F1 progeny:

Monohybrid Ratios:

• Phenotypic Ratio – Tall: Dwarf = 3:1
• Genotypic Ratio – TT: Tt: tt = 1:2:1

Dihybrid Cross:

Dihybrid Ratios:

• Phenotypic Ratio:

Yellow Round : Yellow Wrinkled : Green Round : Green Wrinkled = 9:3:3:1

• Genotypic Ratio:

RRYY: RRYy: RRyy : RrYY : RrYy : Rryy : rrYY : rrYy : rryy =  1:2:1:2:4:2:1:2:1

The following laws of inheritance were derived by Mendel from these crosses:

1. Law of Dominance: 

• It states that in a pair of alleles for a particular gene, one allele can suppress the expression of the other allele. 
• In such a case, only the dominant allele will be expressed as the phenotype, and the recessive allele will be expressed only in the absence of the dominant allele.

2. Law of Segregation:

• It states that during the formation of gametes (sperm and eggs), the two alleles for a given gene separate, or segregate so that each gamete carries only one allele for each gene.
• It is a universal law

3. Law of Independent Assortment:

• During the formation of gametes, the segregation of one pair of alleles is independent of the segregation of another pair.
• As a result, different combinations of traits may be seen in the offspring as compared to the parent. 

Modified Mendelian Inheritance

The genetic principles discovered after Mendel, which do not follow Mendel’s laws of heredity are called non-Mendelian inheritance principles. 

1. Incomplete Dominance:

• The alleles of a gene do not show complete dominance over each other.
• Rather, the phenotype produced in heterozygous individuals is the result of a partial expression of both the alleles – it is intermediate to the two extremes of the trait. 
• E.g. snapdragon flowers (Antirrhinum majus)

   Other examples : Flower colour in Mirabilis jalapa (4 o’clock plant), Feather colour of Andalusian chickens

• ∴ Phenotypic ratio = Red:Pink:White = 1:2:1

2. Co-dominance/Mosaic Inheritance:
   • No dominant allele
   • Both alleles expressed equally and independently – so no intermediate phenotype is seen here
   • E.g. ABO blood grouping in humans – the alleles for A & B antigens are co-dominant, and as a result, individuals with AB blood group have both antigens expressed on their RBCS.
   • Phenotypic ratio: Blood groups A:AB:B = 1:2:1
   • Another example is coat colour of shorthorn cattle (Red and white produce roan-coloured cattle, as both are expressed)
3. Multiple Alleles:
   • More than two alternate forms of a gene (alleles) exist for a single trait at the same locus within the homologous chromosomes
   • No crossing over in multiple alleles 
   • Wild type allele is nearly always dominant over the mutant allele
   • More than two alleles exist for a particular gene within a population, contributing to genetic diversity 
   • E.g. 

Blood groups in humans I^A (A antigen), I^B (B antigen), i (no antigen, recessive)

• Coat colour in rabbits

The alleles include:

C (full color, dominant)

c^ch (chinchilla, partial color)

c^h (Himalayan, temperature-sensitive color)

c (albino, recessive)

4. Complementary gene action:
   • Pair of genes working together to produce a particular phenotype 
   • Only two phenotypes seen instead of 4 
   • If:
     • Homozygous dominant       –   Dominant phenotype
     • Both genes heterozygous       – Dominant phenotype
     • Heterozygous gene + homozygous recessive – Recessive phenotype
     • Both homozygous recessive       – Recessive phenotype
   • Atleast one dominant allele should be present for each gene controlling the trait to get the dominant phenotype
   • In a dihybrid cross with complementary genes –
     • Phenotypic Ratio of F2 generation – Dominant:Recessive = 9:7
     • E.g. Flower colour in sweet pea
       • C_P_ – purple flowers 
       • ccP_ / C_pp / ccpp – white flowers
5. Epistasis:
   • One gene (at one locus) masks or modifies the expression of another gene at a different locus
   • Types of epistasis:
     • Dominant Epistasis:
       • Description: A dominant allele at one locus masks the expression of alleles at another locus.
       • Phenotypic Ratio: 12:3:1
       • Example: In summer squash, the dominant allele for white color (W) masks the expression of alleles for yellow or green color.
     • Recessive Epistasis:
       • Description: A recessive allele at one locus masks the expression of alleles at another locus.
       • Phenotypic Ratio: 9:3:4
       • Example: In Labrador retrievers, the presence of two recessive alleles (ee) at one locus masks the expression of black or brown coat color, resulting in yellow coat.
     • Duplicate Dominant Epistasis:
       • Description: Either of two dominant alleles at different loci can produce the same phenotype.
       • Phenotypic Ratio: 15:1
       • Example: In some plants, if either of two genes has a dominant allele, the trait is expressed.
     • Duplicate Recessive Epistasis:
       • Description: Two recessive alleles at either of two loci can produce the same phenotype.
       • Phenotypic Ratio: 9:7
       • Example: In sweet peas, two recessive alleles at either of two loci result in white flowers, while having at least one dominant allele at both loci results in colored flowers.
     • Dominant Inhibitory Epistasis:
       • Description: A dominant allele at one locus inhibits the expression of alleles at another locus.
       • Phenotypic Ratio: 13:3
       • Example: In some plants, a dominant allele can suppress pigment production regardless of other gene expressions.
     • Polymeric Gene Interaction:
       • Description: Two dominant alleles at different loci intensify the phenotype or create a new variation.
       • Phenotypic Ratio: Varies depending on specific gene interactions.
   • Examples of Epistasis:
     • Coat Color in Mice: The agouti gene determines coat color patterns, but another gene can cause albinism, masking the agouti gene’s effects.
     • Albinism in Humans: A gene mutation can prevent melanin production, overriding other genes that would determine hair or skin color.
     • Flower Color in Peas: Two genes are involved in pigment production; both must have dominant alleles for the color to be expressed.

Summary:

S. No.	Type	Phenotypic ratio in F2
1	Incomplete dominance	3:1
2	Codominance	1:2:1
3	Complementary gene action	9:7
4	Dominant Epistasis	12:3:1
5	Recessive Epistasis	9:3:4
6	Duplicate dominant	15:1
7	Duplicate recessive	9:7
8	Dominant Inhibitory	13:3
9	Supplementary genes	9:3:4

Sex-linked, sex-limited and sex-influenced inheritance

Sex-Linked Inheritance

Types of Sex-Linked Inheritance:

1. X-Linked Inheritance:
   • Description: Traits determined by genes located on the X chromosome. Since females have two X chromosomes (XX) and males have one X and one Y chromosome (XY), X-linked traits are more commonly expressed in males.
   • Examples:

1. X-linked recessive disorder

• Red-Green Color Blindness: A recessive disorder where affected individuals have difficulty distinguishing between red and green colors.
• Hemophilia: A recessive disorder that impairs blood clotting, leading to excessive bleeding.
• Duchenne Muscular Dystrophy: A severe form of muscular dystrophy caused by mutations in the dystrophin gene on the X chromosome.

2.     X-linked dominant disorders:

• Incontinentia Pigmenti: A disorder affecting the skin, hair, teeth, and central nervous system

2. Y-Linked Inheritance:
   • Description: Traits determined by genes located on the Y chromosome. These traits are only passed from father to son, as only males carry the Y chromosome.
   • Examples:
     • Hypertrichosis (Hairy Ears): A condition characterized by excessive hair growth on the ears.
     • Spermatogenesis Genes: Genes involved in the production of sperm, which are passed directly from father to son

Characteristics of Sex-Linked Inheritance

• X-Linked Recessive Traits:
  • More common in males because they have only one X chromosome. A single recessive allele on the X chromosome will result in the trait being expressed.
  • Females can be carriers if they have one recessive allele but typically do not express the trait unless they have two recessive alleles.
  • Criss-Cross Inheritance: X-linked recessive genes are transmitted from an affected male to all his daughters (carriers) and then to half of his grandsons through those daughters.
• X-Linked Dominant Traits:
  • Can be expressed in both males and females if they inherit the dominant allele.
  • Affected males will pass the trait to all their daughters but none of their sons.
• Y-Linked Traits:
  • Only affect males and are passed directly from father to son.
  • Traits include those related to male development and fertility.

Sex-Limited Inheritance

Definition:
Sex-limited inheritance refers to traits that are expressed in only one sex, even though the genes responsible for these traits are present in both sexes. These traits are typically influenced by the hormonal environment specific to one sex.

Examples:

• Milk Production in Cattle: Only female cattle produce milk, even though both males and females carry the genes for milk production.
• Egg Production in Poultry: Only hens lay eggs, although roosters also carry the genes for egg production.
• Prolificacy in Rabbits, Swine, and Goats: Traits related to reproductive efficiency are expressed only in females.
• Plumage in Peacocks: Males display elaborate tail feathers, while females do not.
• Beard in Men: Beard growth is a sex-limited trait expressed only in males.
• Breast Development in Women: Breast development is typically seen in females, although hormonal imbalances can cause breast development in males.

Key Points:

• Expressed in only one sex.
• Presence of genes in both sexes, but expression is limited to one due to hormonal differences.
• Examples include milk production, egg production, and secondary sexual characteristics like beard growth and breast development.

Sex-Influenced Inheritance

Definition:
Sex-influenced inheritance involves autosomal traits (not located on sex chromosomes) but are expressed differently in males and females. The expression of these traits is influenced by the sex of the individual, often due to hormonal differences.

Examples:

• Baldness: The gene for baldness is dominant in males but recessive in females. A male needs only one allele for baldness to express the trait, while a female needs two.
• Horn Development in Sheep: In the Dorset breed, both sexes are horned, while in the Suffolk breed, both sexes are hornless. The expression of horns is influenced by the genetic background and sex of the sheep.

Key Points:

• Autosomal traits that are expressed differently in males and females.
• Expression is influenced by the hormonal environment.
• Examples include baldness and horn development in sheep.

Sex determination

Sex determination in animals is a fascinating and complex process that varies significantly across different species. Here are some detailed notes on the primary mechanisms of sex determination in animals:

Chromosomal Sex Determination

Chromosomal sex determination is one of the most common methods and involves specific combinations of sex chromosomes that determine an individual’s sex.

• XY System: This is prevalent in mammals, including humans, where females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The presence of the Y chromosome, which carries the SRY gene, leads to the development of male characteristics.
• ZW System: Found in birds and some reptiles, this system is the opposite of the XY system. Females are ZW, and males are ZZ. The W chromosome plays a crucial role in determining female characteristics.
• XO System: Used by some insects like grasshoppers, where females are XX, and males are XO (having only one X chromosome and no Y).

Haplodiploidy: Seen in species like bees and ants, where unfertilized eggs develop into males (haploid), and fertilized eggs develop into females (diploid).