BIOLOGY 1111
PATTERNS OF INHERITANCE
CHAPTER 11
Instructor Terry Wiseth
GREGOR MENDEL
Czech Republic 1850
teacher of math
amateur plant breeder
GREGOR MENDEL
combined talents in plant breeding and statistics
used pea plants to track observable characteristics from one generation to the next
looking for patterns of inheritance to explain the process of heredity
GENES
units of information about specific traits
each gene has a special location locus on a chromosome
DIPLOID CELLS
have a pair of genes for each trait on a pair of homologous chromosomes
slight molecular differences in versions of the gene pair are called alleles
ex: trait = flower color
versions = purple, white
HOMOZYGOUS AND HETEROZYGOUS
if alleles are the same version they are said to be homozygous
if alleles are different they are said to be heterozygous
DOMINANT AND RECESSIVE
an allele is dominant when its effect on a trait masks that of another
upper case for dominant---A
an allele if recessive when its effect on a trait is masked by another
lower case for recessive---a
GENETIC TERMS
homozygous dominant ( AA )
homozygous recessive ( aa )
heterozygous ( Aa )
GENOTYPES AND PHENOTYPES
genotypes
phenotypes refer to an individuals observable traits
MENDEL’S EXPERIMENT
cross between a true breeding white flower and a true breeding purple flower
result of the cross are all purple flowers
thus: Purple is dominant over white
OTHER DOMINANT TRAITS
other observable dominant characteristics in pea plants
round seed dominant over wrinkled seed
yellow seed dominant over green seed
tall plant dominant over dwarf plant
MENDEL’S CROSS
P
1true breeding white flower cross with true breeding purple flower
aa X AA
F1 Aa
(purple)
F2 Aa X Aa
F2
MONOHYBRID CROSSF2 Aa X Aa
Aa
Aa
aa AA
3:1 ratio of dominant characteristic over the recessive characteristic
PUNNETT SQUARE
probability table used to visualize the possible gene combinations in a zygote
THEORY OF SEGREGATION
1) diploid cells have pairs of genes on pairs of homologous chromosomes
2) during meiosis the two genes separate from each other
3) genes end up in different gametes
THEORY OF INDEPENDENT ASSORTMENT
during meiosis:
the gene pairs on homologous chromosomes tend to be sorted into one gamete or another
this assortment occurs independently of how gene pairs on other chromosomes are sorted out
INDEPENDENT ASSORTMENT
allows large number (3.5 billion) of possible genotypes in humans
DIHYBRID CROSSES
model of tracking how two pairs of genes are assorted into gametes
ex: flower color height
purple flower---A tall------ B
white flower-----a dwarf---b
DIHYBRID GENOTYPES
genotype
AA purple
aa white
Aa purple
BB tall
bb dwarf
Bb tall
DIHYBRID GENOTYPES
genotypes
AaBB ?
AABb ?
aabb ?
AABB ?
AaBb ?
AAbb ?
aaBB ?
P1 DIHYBRID CROSS
AABB X aabb
F1
all AaBb (purple tall)
F2 DIHYBRID CROSS
9:3:3:1 ratio
9 dominant, dominant
3 dominant, recessive
3 recessive, dominant
1 recessive, recessive
DOMINANCE
one genes expression is inhibited by another’s expression
1) incomplete dominance
2) codominance
INCOMPLETE DOMINANCE
heterozygous phenotype expresses a phenotype somewhere between the two homozygous phenotypes
ex: snapdragons
Red X White
RR X WW
all offspring are RW (pink)
CODOMINANCE
both genes are expressed
ex: blood type
phenotypes A, B, AB, O
3 alleles of the gene for blood type expression
IA, IB codominant
i recessive
ABO BLOOD TYPING
if blood of two people with different "self" markers mixes during transfusions:
immune system of the receiving individual will recognize and attack the foreign RBC
can result in death
PLEIOTROPY
a single gene may influence unrelated traits
ex: a mutated gene for hemoglobin
results in sickle cell anemia if both recessive genes are inherited
SICKLE CELL ANEMIA
causes the shape of red blood cells to be sickle shaped
unable to transport oxygen properly
tissues in the body become anemic (oxygen starved)
EPISTASIS
traits may result from interactions among two or more gene pairs
ex: two alleles from one gene can mask alleles of another
expected phenotypes may not appear
ALBINISM
albino trait carried by recessive genes
EPISTASIS
common for color of fur or skin
ex: Labrador retrievers
phenotypes Black, Brown (chocolate), Yellow
COAT COLOR IN LABS
variations in:
1) amount of melanin (black pigment)
2) distribution of the pigment in the hair shaft
B= black b= brown
E= permits deposition e= blocks deposition
F2 DIHYBRID CROSS
BbEe X BbEe
BLACK PHENOTYPE (9)
BROWN PHENOTYPE (3)
YELLOW PHENOTYPE (4)
9 BLACK, 3 BROWN, 4 YELLOW
COMB SHAPE IN POULTRY
some cases of epistasis bring about phenotypes that neither pair can
produce alone
ex: two genes R and P that produce comb shape in poultry
COMB SHAPE IN POULTRY
single comb
walnut comb RRPP,RRPp, RrPP, RrPp
pea comb rrPP, rrPp
rose comb RRpp, Rrpp
SINGLE AND WALNUT COMB
ROSE AND PEA COMB
RrPp X RrPp
PHENOTYPE RESULTS
9 walnut comb
3 rose comb
3 pea comb
1 single comb
CONTINUOUS VARIATION
generally, the individuals of a population show a range of small
differences in most traits
the greater the number of genes and environmental factors that can influence a trait, the more continuous are the distribution of the versions of the trait
CONTINUOUS VARIATION
ex: eye color . height
statistical analysis shows a typical "bell-shaped" curve of the traits distribution
ENVIRONMENTAL EFFECTS ON PHENOTYPE
the environment can influence the expression of genes
ex: water buttercups
submerged leaves have a different appearance than those above the water surface
WATER BUTTERCUP
ENVIRONMENTAL EFFECTS ON PHENOTYPE
ex: Himalayan rabbit and Siamese cat
carry a heat sensitive version of the enzymes used in melanin production
HEAT SENSITIVE GENE
enzyme is less active at warm body regions (body)
fur grows in lighter
enzyme is most active in cooler regions (ears, nose, feet)
fur grows in darker
CONCLUSION
individuals of a population show degrees of variation in their traits due to:
1) gene mutations
2) cumulative gene interactions
3) environmental effects on genes
END CHAPTER 11