• Inheritance: the acquisition of traits by their parent to offspring

• There is evidence for particulate inheritance

  • determines heredity traits are transmitted in particles from generation to generation

• Gene: unit of heredity

15.1 Mendel’s Laws of Inheritance

• Gregor Mendel studied Pea plants, discovered hybrids

The common garden pea

• Genetic variation

  • peas have many varieties, which differ in characteristics

    • called characters

  • Each character has differing forms known as variants

  • Traits are identifiable characteristics of an organism

• Self fertilization

  • female gamete is fertilized by a male gamete from the same plant

  • makes it easier to produce plants with a certain trait

    • true breeding line/strain: when lineage of plants exhibit the same trait after generations of self-breeding

• Hybridization

  • Mendel wanted to breed hybrids via hybridization

    • occurs via cross-fertilized experiments

Mendel’s Law of Segregation

• Single factor cross: when experimenter followed variants of one character

• P generation: parental generation

• F₁ generation: offspring generation

• When parents differ in a single characteristic, offspring is called monohybrid

• Three important ideas about traits

  • traits may exist in two forms: dominant and recessive

  • individuals carry two genes for a given character

  • two alleles separate during gamete formation

• Dominant and recessive traits

  • dominant refers to displayed traits, recessive refers to hidden traits

• Genes and alleles

  • genetic determinants of traits are unit factors passed from generation to generation

• Segregation of alleles

  • Mendel’s law of segregation: states that two alleles of the same gene separate from each other

Genotype and Phenotype

• Genotype: the genetic composition

• Phenotype: characteristics that appear due expression of genes

• Homozygous: an individual that carries identical copies of an allele

  • Can be homozygous recessive or homozygous dominant

• heterozygous: carries different allele of a gene

  • So both recessive and dominant

Inheritance pattern

• Two-factor cross: when experimenter follows inheritance of two characteristics

• Two possible patterns

  • genes are linked in some way, so variants occur in a unit

  • transmission of genes may be independent, alleles are passed randomly

• Dihybrids: organisms that express big traits

  • led to Mendel’s law of independent assortment

    • alleles sort independently of each other

15.2 Chromosome Theory of Inheritance

• Modern version of chromosome theory of inheritance follows

  • chromosomes contain DNA, genes found in chromosomes

  • chromosomes are replicated and passed to offspring

  • nucleus of diploid cell contain two sets of chromosomes

  • meiosis lets homozygous pairs separated from each other

  • gametes are haploid cells

Segregation of homologous chromosomes during meiosis

• lows: physical location of a gene

• when a cell prepares to divide, homologs replicate forming sister chromatids

  • each chromatid carries one of the alleles

• when meiosis II occurs sister chromatids separate into haploid cells

  • so each haploid cell has one copy of the two original alleles

Mendel’s Law of Independent Assortment

• During Metaphase I, two pairs of chromosomes randomly align on metaphase plate, which leads to random genes paired with each other

  • this is independent assortment

15.3 Pedigree Analysis of Human Traits

• How geneticists track genetic diseases in humans

  • Morally wrong to breed humans against their will to see whether genetic traits are recessive or dominant

• Diseases can be from mutations or form the wild type allele

  • The most common phenotype in an population

15.4 Variations in Inheritance patters and their molecular phases

• Mendelian Inheritance: inheritance patterns of genes that segregate and assort independently

• Simple Mendelian Inheritance: Single gene that has two variants controlled by dominant and recessive traits

Protein function explains dominance traits

• Loss of function alleles: mutations that produce recessive alleles more likely decrease protein function

• In simple dominant recessive relationship, dominant/wildtype allele can suppress it

• Not all recessive alleles are harmful to phenotype

• Pedigree analysis: studying human genetics through family tree

• used to understand human genetic diseases

• Genes exist in two different forms

  • wild type allele: the most common allele in a population

  • mutant allele: some people inherit diseases, not as likely to occur

Recessive alleles that cause diseases affect phenotype

• Pleiotropy: mutation in a single gene has multiple effects on phenotype

• occurs for three different reasons

  • expression could affect cell function in more than one way

  • May be expressed in different cell types

  • may be expressed in different stages

• often times, expression of genes are pleiotropic

Incomplete Dominance results in intermediate phenotype

• Intermediate dominance: two dominant genes compete, meet halfway in phenotype

Environment affects phenotype

• Norm of reaction: phenotype range that shows in a certain environment

15.5 Sex Chromosomes & X-linked inheritance patterns

Sex Chromosomes are due to presence of sex chromosomes

• Several mechanisms determination have been found in different species of animals

  • XY system

    • the one found in humans

      • males are xy, females xx

  • XO System

    • occurs in some insects

    • presence of Y doesn’t mean male

    • gender depends on ratio of X to Y

      • less X chromosomes means male

  • ZW System

    • birds and fish

    • males are ZZ, female are ZW

  • Haplodiploid System

    • found in bees

      • makes come from unfertilized eggs

        • are haploid due to half chromosomes

  • Environmental systems

    • in certain reptiles and fish, gender is controlled by environmental factors

In humans X linked recessive traits are more likely to occur

• X chromosomes are larger and carry more protein coding genes than Y chromosomes

• X-Linked genes: genes found in X-chromosome only

• Sex-linked gene: genes that are only found in one chromosome, not the other