Mendelian Genetics and Inheritance

• Principle of inheritance and variations is a fundamental concept in biology that explains how traits are passed from parents to offspring and how these traits can vary within a population.
• It includes Mendelian genetics, chromosomal inheritance, and trait inheritance patterns.

Mendelian Genetics

• Mendelian genetics, named after Gregor Mendel, is a set of principles that explain how traits are inherited through generations.
• Mendel conducted his experiments using pea plants in the 19th century.
• He proposed the laws of inheritance based on his observations.
• Mendel's work laid the foundation for the field of genetics.

Mendel's Laws of Inheritance

Law of Segregation

• The Law of Segregation states that each individual has two alleles for each trait.
• These alleles segregate (separate) during gamete formation.
• Each gamete carries only one allele for each trait.
• Offspring inherit one allele from each parent, resulting in the diploid condition.

Law of Independent Assortment

• The Law of Independent Assortment states that the alleles of different genes assort independently of one another during gamete formation.
• This law applies when genes for different traits are located on different chromosomes or are far apart on the same chromosome.
• It results in a greater variety of genetic combinations in the offspring.

Law of Dominance

• The Law of Dominance states that in a heterozygote, one allele will mask the effect of the other allele.
• The allele that masks the other is called the dominant allele.
• The allele whose effect is masked is called the recessive allele.
• Only the dominant trait will be expressed in the phenotype of the heterozygote.

Monohybrid Cross

• A monohybrid cross involves the inheritance of a single trait.
• Mendel crossed true-breeding pea plants with different traits.
• The F1 generation showed only the dominant trait.
• The F2 generation showed a 3:1 phenotypic ratio of dominant to recessive traits.
• This demonstrated the principles of segregation and dominance.

Dihybrid Cross

• A dihybrid cross involves the inheritance of two different traits.
• Mendel crossed true-breeding pea plants with different combinations of traits.
• The F2 generation showed a 9:3:3:1 phenotypic ratio.
• This demonstrated the principle of independent assortment.

Genetic Terminology

• Gene: A unit of heredity that codes for a particular trait.
• Allele: A variant form of a gene.
• Genotype: The genetic makeup of an individual (e.g., AA, Aa, aa).
• Phenotype: The observable characteristics of an individual (e.g., tall, short).
• Homozygous: Having two identical alleles for a trait (e.g., AA or aa).
• Heterozygous: Having two different alleles for a trait (e.g., Aa).
• Dominant: An allele that masks the expression of another allele.
• Recessive: An allele whose expression is masked by a dominant allele.

Chromosomal Inheritance

• Chromosomal inheritance describes how genes are passed on through chromosomes.
• It links Mendel's laws to the behavior of chromosomes during meiosis.

Chromosome Theory of Inheritance

• The chromosome theory of inheritance states that genes are located on chromosomes.
• The segregation of chromosomes during meiosis explains the segregation of alleles.
• The independent assortment of chromosomes explains the independent assortment of genes.
• Thomas Hunt Morgan and his colleagues provided experimental evidence for this theory using fruit flies (Drosophila melanogaster).

Sex-Linked Inheritance

• Sex-linked inheritance involves genes located on sex chromosomes (X and Y chromosomes).
• In humans, females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
• Genes on the X chromosome can show different inheritance patterns in males and females.
• Examples include hemophilia and color blindness, which are more common in males.

Linkage and Recombination

• Linked genes are genes located close together on the same chromosome.
• They tend to be inherited together.
• Recombination, or crossing over, can separate linked genes.
• It occurs during meiosis when homologous chromosomes exchange genetic material.
• The frequency of recombination is proportional to the distance between genes on a chromosome.
• This principle is used to create genetic maps.

Trait Inheritance Patterns

• Trait inheritance patterns describe how specific traits are passed from parents to offspring.
• Patterns can be simple or complex, depending on the genetic mechanisms involved.

Autosomal Dominant Inheritance

• Autosomal dominant inheritance involves genes located on autosomes (non-sex chromosomes).
• Only one copy of the dominant allele is needed for the trait to be expressed.
• Affected individuals usually have at least one affected parent.
• Examples include Huntington's disease and achondroplasia.

Autosomal Recessive Inheritance

• Autosomal recessive inheritance involves genes located on autosomes.
• Two copies of the recessive allele are needed for the trait to be expressed.
• Affected individuals usually have unaffected parents who are both carriers of the recessive allele.
• Examples include cystic fibrosis and sickle cell anemia.

X-Linked Dominant Inheritance

• X-linked dominant inheritance involves genes located on the X chromosome.
• Only one copy of the dominant allele on the X chromosome is needed for the trait to be expressed.
• Affected males pass the trait to all their daughters and none of their sons.
• Affected females (if heterozygous) pass the trait to half of their sons and half of their daughters.

X-Linked Recessive Inheritance

• X-linked recessive inheritance involves genes located on the X chromosome.
• Two copies of the recessive allele are needed for the trait to be expressed in females.
• Only one copy of the recessive allele is needed for the trait to be expressed in males.
• Affected males inherit the allele from their mothers.
• Affected females must inherit the allele from both parents.
• Examples include hemophilia and color blindness.

Y-Linked Inheritance

• Y-linked inheritance involves genes located on the Y chromosome.
• Only males are affected. Affected males pass the trait to all their sons.
• The Y chromosome has relatively few genes, so Y-linked traits are rare.

Non-Mendelian Inheritance

• Not all inheritance patterns follow Mendel’s laws.
• These are called non-Mendelian inheritance patterns.

Incomplete Dominance

• Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygous phenotypes.
• For example, in snapdragons, a cross between a red-flowered plant and a white-flowered plant produces pink-flowered plants.

Codominance

• Codominance occurs when both alleles in a heterozygote are fully expressed.
• For example, in human blood types, individuals with the AB blood type express both the A and B antigens on their red blood cells.

Polygenic Inheritance

• Polygenic inheritance occurs when a trait is controlled by multiple genes.
• Each gene contributes a small amount to the phenotype.
• This results in a continuous range of phenotypes.
• Examples include height, skin color, and intelligence.

Pleiotropy

• Pleiotropy occurs when one gene affects multiple traits.
• For example, Marfan syndrome, caused by a mutation in a single gene, affects the skeletal, cardiovascular, and ocular systems.

Environmental Effects

• Environmental factors can influence the expression of genes.
• For example, the color of hydrangea flowers depends on the pH of the soil.
• Genotype and environment interact to determine the phenotype.