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Introduction to Genetics, with a Focus on Human Genetics. Mendel’s Pea Plants. Genetics Terms. Gene and Alleles Homozygous Heterozygous Genotype Phenotype Dominant Recessive. Homologous Chromosomes vs. Sister Chromatids. Gene – section of DNA that codes for a protein

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genetics terms
Genetics Terms
  • Gene and Alleles
  • Homozygous
  • Heterozygous
  • Genotype
  • Phenotype
  • Dominant
  • Recessive

Gene – section of DNA that codes for a protein

Alleles – different molecular versions of a given gene (trait)

  • Found at the same location (locus) on homologous chromosomes

Homozygous – alleles for a given gene (trait) are the same

Heterozygous – alleles for a given gene (trait) are different


Genotype – genetic makeup of an organism

    • A pea plant is homozygous white.
  • Phenotype – physical expression of the genotype
    • The pea plant’s flowers are white.

Dominant allele – the version of the trait that is expressed in a heterozygous individual

  • Recessive allele - the version of the trait that doesn’t impact the phenotype in a heterozygous individual
    • only expressed in an individual who is homozygous for the allele
writing genotypes
Writing Genotypes
  • A given trait is assigned a letter.
    • The dominant version of a trait is shown as the upper-case version of the letter.
    • The recessive version of the trait is shown in lower-case.
      • Pick letters that look different upper and lower-case!

My genotype for eye color: Bb

letter code: brown eyes (B), blue eyes (b)

My phenotype: I have brown eyes.

  • Which is version of the trait is dominant?
  • Am I heterozygous of homozygous for eye color?
Our focus will be on single gene disorders
    • A defect in a single gene results in the disorder.
  • Other genetic disorders are chromosomal disorders.
    • Extra chromosomal material or missing chromosomal material results in the disorder
      • Karyotype analysis usually shows this type of genetic disorder.
      • Down’s syndrome and cri du chat are examples of chromosomal disorders.

Karypotype for female with Down Syndrome

Down Syndrome results from trisomy at chromosome #21.

chromosomes and gender
Chromosomes and Gender
  • The first 22 pairs of chromosomes are called the autosomal chromosomes
  • The sex chromosomes determine gender.

- Human females have two X chromosomes. - Males have one X and one Y chromosome.

single gene autosomal disorders
Single Gene Autosomal Disorders
  • Autosomal traits – the genetic information for the trait is found on chromosome 1-22, not on one of the sex chromosomes
  • Autosomal disorders may be:
    • Recessive
    • Co-dominant – blood type
    • Dominant
writing pedigrees
Writing Pedigrees
  • Create a pedigree for the family described and determine the genotypes of as many individuals as possible.
  • Father – brown eyes, mother – blue eyes
  • Children:
    • Daughter – brown eyes, married to blue eyed man
    • Daughter – brown eyes, married to brown eyed man; two boys have brown eyes
    • Son – blue eyes, married to blue eyed woman
  • Circle = female
  • Square = male
  • …….see board and links below
autosomal recessive inheritance
Autosomal Recessive Inheritance
  • Either parent can carry the recessive allele on an autosomal chromosome.
  • Heterozygotes are symptom-free; homozygotes have the disorder.
  • Two heterozygous parents have a 25 percent chance of producing a child with the disorder.
  • When both parents are homozygous, all children can be affected.
co dominance
  • More than 2 alleles for a given trait
  • Some versions of the gene are dominant over others
    • But they are not dominant over all of the alleles
  • Both dominant alleles are expressed in heterozygotes
writing blood type alleles 2 methods
Writing Blood Type Alleles2 methods
  • There are 3 alleles for blood type: A, B, O
    • A (IA ) and B (IB ) are co-dominant
    • Both A and B are dominant over the allele for O (i) blood
  • Writing alleles for blood type
    • A = IA
    • B = IB
    • O = i
blood types and genotypes
Blood Types and Genotypes
  • Type A Blood






IA i



blood types and genotypes23
Blood Types and Genotypes
  • Type B Blood



IB i



blood types and genotypes24
Blood Types and Genotypes
  • Type AB Blood

Type O Blood




i i

  • Dominant disorders – the allele for the disorder is dominant and the healthy version of the trait is recessive
    • The genetic form of Huntington’s disease is dominant to being healthy.
    • Some forms of breast cancer are dominant
      • More to come on this
sex linked traits
Sex-linked Traits

X-linked traits: genetic information for the trait is located on the X chromosome

  • no genetic info about the trait on the Y chromo
  • Examples: Hemophilia, color blindness, Duschenne muscular dystrophy

Y-linked traits (not many known)

  • Allele for hairy ears is Y-linked
genetics and breast cancer
Genetics and Breast Cancer
  • An inherited form of breast/ovarian cancer is an autosomal dominant condition
    • Occurs as a result of mutations in BRCA1 or BRCA2 genes
      • BRCA 1 - gene on chromosome 17
      • BRCA 2 - gene on chromosome 13

BRCA 2 Chromosome 13

BRCA 1 Chromosome 17

function brca genes
Function BRCA Genes
  • The BRCA genes seem to code for tumor suppressor proteins
    • Needed to keep cells from becoming cancerous
  • BRCA gene products also play a role in:
    • DNA repair
    • Cell Cycle control
    • And other functions
heterozygous individuals
Heterozygous Individuals
  • Theoretically, most individuals with the BRCA 1 or 2 gene will also carry a normal allele on the other paired chromosome.
  • Normal allele should make functional BRCA protein and it should act as a tumor suppressor…….
heterozygous individuals33
Heterozygous Individuals
  • HOWEVER…..
    • In most breast and ovarian cancers studied in those carrying the mutation the normal allele is deleted or loses function, resulting in loss of all function of the BRCA 1 or 2 proteins.
brca1 brca2
    • Over 2000 different mutations have been identified in the BRCA genes
    • Most BRCA1 and BRCA2 mutations are predicted to produce a truncated protein product, and a loss of protein function.
  • Not all have the same cancer risk
    • The cancer risk associated with each mutation depends on the impact of the mutation on the protein’s structure and function.
incidence of brca genes
Incidence of BRCA genes
  • BRCA1
  • 3.5% Hispanic,
  • 1.3% to 1.4% African American,
  • 0.5% Asian American,
  • 2.2% to 2.9% non-Ashkenazi Caucasian, and
  • 8.3 % to 10.2% Ashkenazi Jewish.
  • BRCA2
  • 2.6% African American and
  • 2.1% Caucasian
increased risk of cancer
Increased Risk of Cancer
  • Individuals with one copy of either mutated gene (BRCA1 and BRCA2) are at increased risk of breast and/or ovarian cancer.
    • ~60% lifetime risk breast cancer
    • ~10-40% lifetime risk ovarian cancer
alexandra best s story
Alexandra Best’s Story
  • Alexandra Best’s mother and grandmother died from breast cancer.
  • After visiting the doctor to voice her concerns, she discovered that researchers had traced the origins of her family's history of breast cancer back to three Italian brothers who emigrated to England in the 1850s.
  • Thirty-one members of her family had developed the disease since then - and the researchers said she was likely to be carrying the gene.
what she did
What she did.
  • Alexandra was tested for the BRCA gene and found to carry it.
    • She decided to have a preventative mastectomy
    • During preparation for the operation a cancerous lump was found in her breast.
      • Lead to chemotherapy after a double mastectomy.
Alexandra’s extended family was tested and 11 were found to carry the BRCA gene
    • 8 have had preventative surgery

Family positive for BRCA 1 deleterious mutation

positive for mutation

** Preventive measure taken

Ethel E. Laird

Breast 28

Death 32

Marjorie Halt

Ovarian 64,69

Death 69

Maryann Laird

Breast 44, 46

Ovarian 63

Death 64

Paula (59)

Breast 48



Breast 38, 40

Death 44

Susan Halt Davis (53)

Breast 49



Linda Halt Nicholson (60)

Steve Halt (57)





Kevin (32)

2 Children

Gina (30)

Kristi (28)

Amber (28)

Jeffrey (40)

No Children


Death 3-5-07

Age 25

Car accident

Jennifer, 22