Bio CH 15
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Wild Type and mutant phenotypes
- tracing genes to chromosomes:
- Included identifying mutant phenotypes
- trait also called the wild type.
- ex: Most common trait is red eyes. Most common
- Alternative trates called mutant phenotypes
- Ex: Less common trait is white eyes
Chromosomal Basis of Sex
- Humans have XX (female) and XY (male)
- -These are two varieties of sex chromosomes
- eggs (x) sperm: (x or y)
- 50/50 chance of offspring sex
- Other chromosomal systems exist
- -variable by species
- Genes are located on either sex chromosomes
- X chromosome has non-sexual characters
- X chromosome larger, other genes present
- --Y chromosome is mostly sex characters
- Fathers pass linked genes to daughters only
- --X from father + X from mother ->XX daughter
- --Y from father + X from mother ->XY son
- More males with sex-linked disorders
- --Females need recessive from both parents
- --Males only need a recessive allele from mom
- ---Can result in Color blindness, muscular dystrophy, hemophilia.
- Females inherit two X chromosomes (XX) However, one becomes inactivated. Inactive X condenses->Barr body. Barr bodies reactivate in ovarian cells. Prevent overexpression of X genes (dosage compensation)
- Mosaic of cells with alternative active Xs--Patchiness of gene expressions. Ex: Toriseshell cats.
- Usually located on same chromosome--Chromosomes passed along as a unit.
- Expected ratio of phenotypes not found. Linked genes do not assor independently where one gene goes, so does the other.
- Genetic recombination can still occur by crossing over can separate the linked genes.
- •Recombinant % data can create gene map
- •Space between genes: recombination %
- –Genes far apart: increased recombination %
- •Greater chance of crossing over separating genes
- –Genes closer: decreased recombination %
- •Lesser chance of crossing over separating genes
- Homologous chromosomes do not separate
- two of the same chromosome->one daughter cell
- No chromosome-> other daughter cell
a result of nondisjunction. there is a wrong chromosome number in zygote.
fertilized egg is 2n+1
Alteration of individual chromosome structure
- Mulitiple Sets of chromosomes=polyploidy
- ex: Triploidy 3n; Tetraploidy: 4n
- Can result from complete nondisjunction
- -All chromosomes to one cell, none other
- Not uncommon in plant kingdom
- -results in speciation
- Rare in higher animals
- Some traits affected by parental source
- --Different effect if gene from mother or father
- --Vs. typically, alleles have equal effect
- New imprinting with each generation
- -Previous imprinting "overwritten" in gametes
- Methylation (-CH3) of cytosine involved
- -Additional functional group silences allele
- One of many epigenic factors
- Exhibit non-Mendelian inheritance patterns
- Ex-DNA in mitochondria, plastids (plants)
- Not distributed by rules of meiosis
- Organelles with genes passed in cytoplasm
- -Cytoplasm comes from egg (maternal)
- Therefor, Genes of plastids are maternal only and genes of mitochondria are maternal only.
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