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Genetics and Inheritance - Video

Genetics: Key Concepts and Inheritance

  • Genetics is the study of heredity. Genes, carried on chromosomes, are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. Key components:

    • Nucleus, Cell, Chromosomes
    • Gene, Protein

  • Genes reside on chromosomes and direct the synthesis of proteins that determine traits.

Mendel and the Pea Plant: Foundations of Inheritance

  • Gregor Mendel (1822–1884) used garden peas to study inheritance because:

    • Peas mature in one season
    • True-breeding lines produce offspring like the parent
    • Hybridizations involved mating two true-breeding plants with different traits

  • Experimental design

    • P generation: true-breeding parents crossed (e.g., White flowers × Violet flowers)
    • F1 generation: first filial hybrids; in Mendel’s experiments, all F1 progeny showed the violet (dominant) phenotype
    • F2 generation: produced by self-fertilization of F1 plants; displayed both violet and white phenotypes in a roughly 3:1 ratio

  • Example numbers from Mendel’s pea cross (phenotype ratio in F2):

    • Violet: 705
    • White: 224
    • Mendel’s observed ratio approximates a 3:1 phenotypic distribution for a single-gene trait
    • Mendel’s actual tall vs short example yields a 3:1 phenotypic ratio in the F2 generation for a monohybrid cross
    • Mendel’s law of segregation and dominance underlie these results

  • Basic cross terminology used by Mendel:

    • P generation = parents
    • F1, F2, etc. = filial generations (offspring)
    • Hybridization of true-breeding plants produces F1 hybrids; self-fertilization of hybrids yields F2

Monohybrid Cross: Crossing Pea Plant Traits

  • Example trait: seed shape (Round vs Wrinkled) or flower color (Purple vs White)

  • Carpel (female part) and stamen (male part) roles in cross-pollination: pollen transfer from male parent to prepared female parent; terms:

    • Carpel, Stamen

  • Phenotypes and genotypes in pea traits follow simple dominant-recessive patterns for many traits when controlled by a single gene.

  • 7 Pea Plant Characteristics (traits commonly studied by Mendel):

    • Seed shape: Round vs Wrinkled
    • Seed color: Yellow vs Green
    • Flower color: Purple vs White
    • Pod shape: Inflated vs Constricted
    • Pod color: Yellow vs Green
    • Stem height: Tall vs Dwarf
    • Flower position: Axial vs Terminal

  • A trait is a variation in the physical appearance of a heritable characteristic.

Mendel’s Law of Segregation and Dominance

  • Law of Segregation: Paired unit factors (genes) segregate equally into gametes; offspring have equal likelihood of inheriting either factor. In a heterozygote, one allele is dominant and the other recessive.

    • Genotype: TT × tt → all F1 are Tt (tall if tall is dominant)
    • F2 cross (Tt × Tt):
    • Genotypes: TT : Tt : tt = 1 : 2 : 1
    • Phenotypes: 3 ext{ tall} : 1 ext{ short}

  • Law of Dominance: In a heterozygote, the dominant allele masks the expression of the recessive allele for that trait.

    • Example: albinism in humans (allele for albinism recessive). Both parents can be carriers (heterozygous) and have affected offspring if the recessive allele is inherited from both parents.

  • Example cross (dominant-recessive):

    • If T = tall (dominant) and t = short (recessive):
    • Genotypes: TT, Tt (tall) vs tt (short)
    • Phenotypes: tall vs short according to genotype

Terminology and Genetic Terms

  • Karyotype: Display of chromosomes in pairs ordered by size and features; humans have 46 chromosomes arranged in 23 pairs.
    • 22 pairs autosomes; 23rd pair sex chromosomes
    • XX = female, XY = male
  • Autosomes vs Sex Chromosomes:
    • Autosomes: 1–22 pairs; same appearance in males and females
    • Sex Chromosomes: 23rd pair; determine sex (XX female, XY male)
  • Sex determination:
    • Males produce sperm with X or Y; females produce ova with X
    • Offspring sex is determined by the father’s sperm
    • Early development is similar until about 7 weeks gestation; presence of Y chromosome initiates male development

Sex-Linked Inheritance and X-Linked Traits

  • X-linked genes are on the X chromosome; Y chromosome carries far fewer genes of this type.

  • Most sex-linked traits are on the X chromosome because the X contains more genes (900–1400 genes on X; 70–200 on Y).

  • Examples and patterns:

    • Drosophila eye color was one of the earliest identified X-linked traits.
    • Hemophilia A is a classic X-linked recessive disorder involving blood clotting defects.
    • Color vision and color blindness often involve X-linked genes.

  • Inheritance patterns:

    • X-linked recessive: more males affected; females are typically carriers unless the condition is severe.
    • X-linked dominant: rare; affects both sexes; males often more severely affected; many affected male zygotes are aborted.

  • Hemophilia A (X-linked recessive) example:

    • Female carrier genotype: XHXh
    • Normal male genotype: XHY
    • Affected male genotype: XhY
    • Offspring outcomes:
    • Carrier female × Normal male: daughters may be carriers or affected depending on inheritance; sons may be affected if they inherit the Xh from mother
    • Diagrams show normal vs hemophiliac outcomes in male and female offspring

  • X-linked dominant traits:

    • Expressed in both males and females who carry the dominant allele
    • Males may be more severely affected due to having only one X chromosome (no normal recessive allele to counteract)
    • Many affected male zygotes may be spontaneously aborted

Phenotypes, Genotypes, and the Link Between Them

  • Genotype vs Phenotype:
    • Genotype: the genetic makeup (e.g., TT, Tt, tt; IA IA, IAIB, IBIB, ii for ABO; XH Xh for hemophilia example)
    • Phenotype: the observable trait (e.g., tall, short; blood type A, B, AB, O; color vision status)
  • Heterozygous vs Homozygous:
    • Homozygous: same alleles on both chromosomes (TT or tt; IAIA; IBIB; ii; XHXH; XhXh if applicable)
    • Heterozygous: different alleles (Tt; IAIB; XHXh)
  • Dominant vs Recessive in practical terms:
    • Dominant allele: expressed in phenotype when present in any copy (as in TT or Tt)
    • Recessive allele: expressed only when two copies are present (tt)

ABO Blood Group System and Allelic Diversity

  • The classic two-allele view is oversimplified. While individuals have two alleles for a gene, population-level alleles can be multiple.
  • ABO system: three alleles in the population: IA, IB, i
    • IA and IB are codominant to each other; both are dominant to i
    • Possible genotypes and phenotypes:
    • IAIA or IAi → type A
    • IBIB or IBi → type B
    • IAIB → type AB (codominant expression of IA and IB)
    • ii → type O
  • Genotype combinations and phenotypes can be summarized as:
    • IAIA (A), IAi (A), IBIB (B), IBi (B), IAIB (AB), ii (O)
  • Population-level diversity allows many two-allele combinations, even though individuals carry only two alleles per gene.

Polygenic Inheritance and Environmental Influences

  • Polygenic inheritance: multiple genes interact to produce a single phenotypic trait
    • Traits often influenced by both genes and environment
    • Examples: eye color, height, skin color, predisposition to many diseases
  • Environmental influences can modify gene expression and phenotype
  • Penetrance: the percentage of individuals with a genotype who actually express the associated phenotype
    • Example: hereditary pancreatitis with penetrance of 80% means 80% of people with the genotype show symptoms; 20% do not

Punnett Squares and Predicting Offspring Genotypes

  • Punnett Square: a diagram used to predict the genotypes of a cross or breeding experiment
  • Example template for a monohybrid cross (dominant vs recessive):
    • Parent genotypes: A a × A a
    • Gametes: A, a from each parent
    • Offspring genotypes: AA, Aa, Aa, aa
    • Genotype ratio: 1:2:1
    • Phenotype ratio: 3:1 (dominant phenotype vs recessive)
  • Example for sex-linked trait (X-linked recessive):
    • Mother: XHXh (carrier), Father: XHY (normal)
    • Punnett Square shows potential offspring: daughters may be carriers or affected; sons may be affected depending on inherited X chromosome

Common Study Questions and Reflections

  • How does codominant inheritance differ from incomplete dominance?
    • Codominance: both alleles are fully expressed in the phenotype (e.g., IAIB → AB blood type)
    • Incomplete dominance: heterozygote shows an intermediate phenotype (e.g., red × white roses yielding pink)
  • If a woman homozygous for color blindness (XbXb) has children with a man with normal vision (XBY), what are the potential phenotypes? (Typically all sons would be color blind if the allele is X-linked recessive and the father contributes Y; daughters would be carriers, not affected unless the mother’s allele is also transmitted.)

Glossary of Key Terms

  • Homologous Chromosome
  • Allele
  • Dominant
  • Recessive
  • Genotype
  • Phenotype
  • Homozygous
  • Heterozygous

Quick Reference: Numerical and Conceptual Highlights

  • 46 chromosomes in humans arranged in 23 pairs (2 autosome sets + sex chromosomes)
  • Autosomes: 22 pairs; Sex Chromosomes: 1 pair (XX female, XY male)
  • Classic Mendelian ratio for a monohybrid cross: Genotype 1:2:1; Phenotype 3:1
  • Mendel’s tall vs short example: Dominant tall (T) over recessive short (t)
    • Cross TT × tt → all Tt (tall)
    • Cross Tt × Tt → Genotypes TT: Tt: tt = 1:2:1; Phenotypes 3:1 tall to short
  • ABO alleles and dominance: IA and IB are codominant; i is recessive to both; Phenotypes A, B, AB, O
  • Sex-linked inheritance: Genes on X chromosome are often expressed differently in males and females due to XY sex determination
  • Penetrance: % of individuals with the genotype who express the phenotype

Note: The above notes summarize the key concepts, terms, and examples presented in the provided transcript. They are organized to function as a comprehensive study aid and reference for genetics-related topics, including Mendelian and non-Mendelian inheritance, chromosome structure, sex determination, and patterns of inheritance.