Chapter 14
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Chapter 14. Mendel and the Gene Idea. Gregor Mendel. Monk Pea Plants  many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants. P Generation (true-breeding parents). . Purple flowers. White flowers. F 1 Generation

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Chapter 14

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Chapter 14

Chapter 14

Mendel and the Gene Idea


Gregor mendel

Gregor Mendel

Monk

Pea Plants  many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants


Experiments

P Generation

(true-breeding

parents)

Purple

flowers

White

flowers

F1 Generation

(hybrids)

All plants had

purple flowers

F2 Generation

Experiments

  • P generation = true breeding parents

    • Purple x White

  • F1 generation = hybrids from P generation cross

    • All purple plants

  • F2 generation = offspring from F1 self-pollination

    • 3:1 ratio of purple to white plants


Mendel s conclusions

Allele for purple flowers

Homologous

pair of

chromosomes

Locus for flower-color gene

Allele for white flowers

Figure 14.4

Mendel’s Conclusions

  • Alternate versions of genes

    • ALLELES

    • Found on Homologous Chromosomes

    • Represented with letters


Chapter 14

Each trait is controlled by 2 alleles – one contributed from each parent


Chapter 14

  • The Law of Segregation

    • The 2 alleles for a trait segregate during gamete formation (meiosis) and end up in different gametes


Chapter 14

  • Mendel’s Other Law:

    • The Law of Independent Assortment

    • Genes located on different chromosomes are inherited independently

    • Exceptions: genes far away on same chromosome (many map units apart) allowing crossing over to occur can be inherited independently

    • * Genes located close together and are usually inherited together are called “linked” ex: red hair, freckles


Chapter 14

  • Some alleles are dominant while others are recessive

    • Dominant alleles “mask” recessive

    • Dominant alleles = capital letters

    • Recessive alleles = lowercase letters


Vocab

Vocab

  • Homozygous = 2 of the same alleles

    • Homozygous Dominant = DD

      • Shows dominant trait

    • Homozygous Recessive = dd

      • Shows recessive trait

  • Heterozygous = 1 dominant, 1 recessive allele

    • Heterozygous = Dd

      • Shows dominant trait and masks recessive

  • Genotype = allele make up (ex: DD)

  • Phenotype = physical trait


Determine probabilities

Determine Probabilities

  • Punnett Squares

    • Aka Monohybrid Cross  1 trait

  • Explains Law of Segregation


Testcross

Testcross

  • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype

    • Is the organism DD or Dd?!

  • Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait


Chapter 14

Dominant phenotype,

unknown genotype:

PP or Pp?

Recessive phenotype,

known genotype:

pp

APPLICATION An organism that exhibits a dominant trait,

such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’s

genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the

unknown genotype is crossed with a homozygous individual

expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

If PP,

then all offspring

purple:

If Pp,

then 1⁄2 offspring purple

and 1⁄2 offspring white:

p

p

p

p

RESULTS

P

P

Pp

Pp

Pp

Pp

P

p

Pp

pp

Pp

pp

  • The testcross

Figure 14.7


Dihybrid cross

Dihybrid Cross

Typical Outcome for 2 heterozygous individuals: 9:3:3:1

  • Shows inheritance of two traits

    • Ex: seed color AND seed shape


Types of dominance

Types of Dominance

  • Complete Dominance

    • Phenotype of Dd is the same as DD

  • Codominance

    • Shows 2 dominant traits

    • Ex: BB = black, WW = white, BW = black AND white

  • Incomplete Dominance

    • Shows blended pattern

    • Ex: CRCR = Red, CwCw = White, CRCW = Pink


Codominance vs incomplete dominance

Codominance vs. Incomplete Dominance


Multiple alleles

Table 14.2

Multiple Alleles

  • Most traits are controlled by 2 alleles (ex: DD)

  • Some traits have more than 2 alleles

    • ABO Blood Type

    • IA, IB, i are the 3 alelles


Pleiotropy

Pleiotropy

  • 1 gene has multiple phenotypic effects

    • Ex: Sickle Cell Anemia


Multiple genes polygenic

AaBbCc

AaBbCc

aabbcc

Aabbcc

AABBCc

AABBCC

AaBbcc

AaBbCc

AABbCc

20⁄64

15⁄64

Fraction of progeny

6⁄64

1⁄64

Multiple Genes (Polygenic)

Ex: Hair, Skin, Eye Color

Additive Effects


Epistasis

Epistasis

Outcome differs from 9:3:3:1

  • A gene at one locus alters the phenotypic expression of a gene at a second locus (masks the other gene)

  • Ex: Laborador Retrievers

    • 2 genes: (E,e) & (B,b)

    • pigment (E) or no pigment (e)

    • pigment concentration: black (B) to brown(b)


  • Multifactorial traits

    Multifactorial Traits

    Controlled by genetics and environment

    Ex: Skin color in humans, hydrangea color

    Nature vs. Nurture


    Pedigrees

    Pedigrees

    Circles = Females

    Squares = Males

    Shaded = Expresses phenotype

    Half-Shaded = Carriers

    Parents joined by horizontal lines

    Offspring listed below parents in birth order


    Human genetic disorders

    Human Genetic Disorders

    • Most are recessive (require homozygous recessive genotype)

      • Carriers = heterozygous individuals that do not show trait, but “carry” the gene for it (Ex: Aa)

      • Cystic Fibrosis: defective chloride channel in cell membranes lead to build up of thickened mucus

      • Tay-Sach’s disease: brain cells cannot metabolize lipids

      • Sickle-Cell Anemia: misshaped blood cells


    Human genetic disorders1

    Human Genetic Disorders

    • Huntington’s Disease: Dominant, degenerative disease of nervous system

    • Achondroplasia: Dominant form of dwarfism (“AA” is lethal)

    • Many disorders are multifactorial

      • Heart disease

      • Cancer

      • Diabetes


    Carrier recognition

    Carrier Recognition

    • Fetal Testing

      • Amniocentesis: withdrawl of fluid from amniotic sac to collect cells for viewing and karyotyping

      • Chorionic villus sampling (CVS): small amount of tissue suctioned from placenta

      • Ultrasound: non-invasive, reveals structures

    • Newborn Screening

      • Blood: test for PKU (Phenylketonuria)

        • Recessive disorder that leads to mental retardation

        • Requires a change in diet


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