GENETICS
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branch of Biology that explains biological variation
GENES and ALLELES
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genes are sequences of nucleotides that code for a specific functional product (structural proteins.
e.g. collagen, keratin, or enzymes. e.g. DNA polymerase, hexokinase, sucrase)
alleles are alternative forms of a gene that have slightly different DNA base sequences as a result
of mutation
genes are located on chromosomes
genes that code for the same products are found on homologous pairs of chromosomes (homologs)
homologs can be identified by size, position of centromere, and banding patterns produced by
stains
KARYOTYPE – chromosomes are arrested at metaphase, placed in a hypotonic solution,
photographed, enlarged, cut out and arranged into pairs by size, shape and banding patterns
GREGOR MENDEL
- Back in the day (1800’s), the prevailing view of inheritance was blending
WRONG
- Bred pea plants for 8 years and mathematically analyzed results
- Peas self-fertilize, so true breeding varieties were available
- Mendel cross-fertilized his pea plants by removing stamens and dusting stigmas with pollen from
another plant
- He studied 7 different characteristics that appeared in “either/or” form
- He started with pure breeding plants – those that produce offspring IDENTICAL to themselves
- Offspring of parents with different characteristics – HYBRIDS
- Monohybrid cross – parents differed in only ONE trait
- P = parental generation
- Offspring of P generation = F1
- F1 self fertilizes = F2
- He believed that a plant inherited two “units” of information for a trait, one from each parent
- First experiments were monohybrid crosses – two parents that are true breeding for a contrasting
trait
- One form of the trait disappeared during the F1 generation, but reappeared in the F2
- F2 showed a 3:1 phenotypic ratio (physical appearance)
- Genes DO NOT blend
- To show this, he did a testcross, where an F1 individual was crossed with homozygous recessive
individuals; a 1:1 ratio of recessive and dominant phenotypes supported his concept of segregation
- Theory of segregation = 2n organisms inherit two genes per trait located on pairs of homologous
chromosomes
- During meiosis, the two genes separate (segregate) from each other such that each gamete will
receive only one gene per trait
- During meiosis, homologous chromosomes separate into different gametes = PRINCIPLE OF
SEGREGATION
- Dominant trait = capital letter
- Recessive trait = lower case
- Genotype = genetic makeup (GG, Gg, gg); if both alleles are the same (GG, gg) HOMOZYGOUS; if
both alleles are different (Gg) HETEROZYGOUS
- Phenotype = physical appearance
- Punnett square = possible parental gametes are places along sides and squares are filled in – gives
possible offspring genotypes that could result from the union of parental gametes (fertilization)
- To study 2 characteristics, dihybrid cross
- F2 phenotype ratios from pure breeding parents = 9:3:3:1
Alleles for one trait on one homologous pair of chromosomes behave independently of other alleles
for another trait located on a different homologous pair of chromosomes =
PRINCIPLE OF INDEPENDENT ASSORTMENT
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Independent Assortment
- Mendel performed dihybrid crosses, predicting that only the dominant trait would appear in the F1;
he wondered if the two traits were inherited together
- Genes located on nonhomologous chromosomes segregate independently of each other and give a
9:3:3:1 phenotypic ratio
- The theory of independent assortment states that during meiosis, each gene of a pair tends to
assort into gametes independently of other gene pairs located on nonhomologous chromosomes
Codominance and Incomplete Dominance
- a dominant allele cannot mask the expression of another, so a phenotype that is a blend or in
between parents = Incomplete Dominance. e.g. - a pure breeding red flower is crossed with a pure
breeding white flower
RR x rr = Rr phenotype = pink
- when BOTH alleles contribute to the genotype of a heterozygote = Codominance. e.g. – AB blood
type – BOTH alleles are expressed in the phenotype, roan cattle = both red and white hair
- ABO blood system = multiple alleles (genes that have more that 2 alleles)
- ABO blood types – both alleles are expressed in heterozygotes
- When more than 2 alleles exist for a given locus, a “multiple allele system”
PROBABILITY AND GENETICS
- geneticists use probability to predict outcomes of crosses
- the chance that 2 or more independent events will occur together is the product of their chances of
occurring separately
Multiple effects of single gene
- expression of alleles at one location can have effects on two or more traits = pleiotropy
- sickle cell anemia
Interactions between gene pairs
- one gene pair can influence other gene pairs, with their combined activities producing some effect
on the phenotype = epistasis
- Labrador retrievers
Continuous variation
- a given phenotype can vary from one individual to the next because of gene interactions and
environmental factors
- e.g. Height and eye color (humans)
Environmental effects on phenotype
- fur on extremities of animals
- Hydrangea
CHROMOSOMES AND HUMAN GENETIC
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Modern genetics came with a rediscovery of Mendel’s work
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Philadelphia chromosome - #9 with a piece of #22 – altered gene specifies an abnormal protein
which triggers uncontrolled cell division of white blood cells
Diploid organisms possess pairs of homologs which are alike in length, shape and information that
they carry
Alleles – slightly different nucleotide sequences in the same gene
Crossing over results in genetic variation
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Sex chromosomes determine gender
Autosomes = chromosomes of the same quantity and type in both sexes
Chromosomes are visualized in a karyotype
Gene locations
- linked genes – are usually inherited together because of their position on the chromosome
- can be disrupted because of crossing over
Recombination patterns
- the closer the loci of two genes, the greater the tendency that they will be inherited together
Human inheritance
- difficult to study
- why?
- Pedigrees – chart that shows genetic connections among individuals
- Analysis of family pedigrees can show inheritance patterns
Inheritance patterns
1. autosomal recessive
- either parent can carry the recessive allele
- heterozygotes are unaffected but carry allele; homozygotes are affected
- usually skips a generation
2. autosomal dominant
- dominant allele is nearly always expressed
- present in every generation
3. X-linked recessive
- mutated gene occurs only on the x chromosome
- heterozygous females are phenotypically normal; males are more often affected because the single
recessive allele on the X chromosome is not masked by a normal copy
Changes in chromosome structure
- duplication = gene sequence is in excess amount
- inversion = alters position and sequence of genes so that gene order is reversed
- translocation = part of one chromosome is transferred to a nonhomologous chromosome as in the
Philadelphia chromosome
- deletion = loss of a chromosome segment
- this can happen during crossing over in Prophase 1 of Meiosis
Changes in chromosome number
- aneuploidy – gametes or cells end up with one extra or one less chromosome than is normal
- polyploidy – presence of three or more of each type of chromosome in gametes or cells ( common
in plants, usually fatal in humans)
- nondisjunction – occurs at anaphase I or anaphase II – failure of homologs or chromatids to
segregate
trisomy – extra chromosome
monosomy – one less chromosome
Changes in the Number of autosomes
Down syndrome = trisomy 21
Changes in the Number of Sex Chromosomes
Turner syndrome
Klinefelter syndrome
XYY
SEX DETERMINATION
- males produce gametes containing either an X or a Y chromosome
- females produce gametes containing the X chromosome
- both of these processes occur during meiosis
- a zygote that is XX will be female, and an XY zygote will be male
SEX LINKAGE
- genes found on the X chromosome are said to be X-linked
- genes found on the Y chromosome are said to be Y-linked
- any trait found on either of the two sex chromosomes is said to be sex-linked
- see the fruit fly Punnett squares on page 222
LINKAGE GROUPS
- since there are many more genes than chromosomes, each chromosome carries hundreds of
different genes
- genes that are on the same chromosome are said to form a linkage group
- these genes are usually inherited together
- crossing over during prophase one of meiosis can produce new combinations of alleles, leading to
non-Mendelian ratios in offspring
- the further apart two genes are on a given chromosome, the greater the likelihood that they will be
separated during crossing over
- the closer two genes are on a given chromosome, the greater the probability that they will be
inherited together (red hair and freckles)
MUTATIONS
- mutation is a change in the DNA of an organism
- germ cell mutations are passed on to the offspring of that organism if the affected gamete is
fertilized (or fertilizes)
- somatic mutations take place in body cells and can affect the organism (these are not passed on to
offspring)
- lethal mutations often cause death before birth
- not all mutations are bad; some are beneficial and give the organism a better chance of survival