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Heredity / Genetics
Early explanations for heredity?
Gregor Mendel published 1865
Consequences of a monk proposing the nature of heredity?
Early 1900’s accuracy recognized
Grew up on a farm
Educated in mathematics (probability) & science
Why
garden
pea plant?
P1 - Pure Breeding homozygous F1
seeds
phenotypes
F2
phenotypes
Round x
wrinkled
All round
5,474 round
2.96 : 1
1,850 wrinkled
Yellow x
green
All yellow
6,022 yellow
2,001 green
3.01 : 1
All gray
705 gray
224 white
3.15 : 1
Full x
constricted
All full
882 full
299 constrict
2.95 : 1
Green x
yellow
All green
428 green
152 yellow
2.82 : 1
Axial x
terminal
All axial
652 axial
207 terminal
3.14 : 1
All long
787 long
277 short
2.84 : 1
Gray x white
pods
stem
Long x short
F2 ratio
Explanation
Experimentation led to three basic conclusions:
1.
2.
3.
inheritance of each trait is determined by "units" or "factors"
(now called genes ) that are passed on to descendents unchanged
an individual inherits one such unit from each parent for each
trait
that a trait may not show up in an individual but can still be
passed on to the next generation.
Law of Segregation:
This says that of a pair of characteristics (e.g. blue and brown eye color) only one
can be represented in a gamete.
Law of Independent Assortment
different pairs of alleles are passed to offspring independently of
each other. The result is that new combinations of genes present in
neither parent are possible.
Gene for yellow seeds (Y), gene for green (y)
P1 = Pure bred cross - YY x yy
x
y
y
Y
Yy
Yy
Y
Yy
Yy
x
Y
y
Y
YY Yy
y
yY
yy
Essential Terms
•Homozygous vs Heterozygous
•Dominant vs Recessive
•Phenotype vs Genotype
•Ratio
•Alele
•Hybrid
•Monohybrid Cross
•Dihybrid Cross
•P1. F1. F2
•Punnet Square
•Pedigree
•Gamete
•Probability
•Product Rule
Human examples of Mendelian Traits
Listed below are human traits that reveal dominance & recessiveness All traits listed are the dominant phenotype
•Tongue Curling
•Widow’s Peak
•Free Ear lobe
•Brown Pigmented Iris
•Hitch Hikers Thumb (45 degree angle)
•Shorter Big Toe (shorter in length than second toe)
•Mid Digital Hair
•Freckles
•Rh Factor
•PTC taster
•Left Thumb Over Top Right - (When Interlocking Fingers of both hands)
•Short Palmar Muscle (Two tendons back of thumb to wrist is dominant, three is recessive)
Non-Mendelian patterns of inheritance
Incomplete Dominance:
Hybrids produce a trait that reveals a
blend of both aleles
R = red W = white
RR = red
WW = White
RW = Pink
Co-Dominance
Hybrids reveals trait of both aleles
R = red W = white
RR = red
WW = White
RW = Roan (red & white colors)
Polygenic inheritance
Multiple pairs of genes all influencing one trait
Yao Ming 7’5”
Earl Boykins 5’5”
Many pairs of genes
influence the height of a
human. Efficiency of
the digestive system,
circulatory system, and
endocrine system - each
being controlled by
many pairs of genes will all impact height.
Multiple Aleles
Traits in which within a population there exists more than two
forms of a gene.
Human blood type - RBC surface proteins must be compatible when
initiating a transfusion. Some people have A proteins on their RBC’s,
others have B proteins., and others have neither.Within the human
population there exists three forms of the gene that controls surface
proteins on RBC’s.
Alele A (makes A proteins) Alele B (makes B proteins) Alele i (makes neither A or B)
Four main blood types: (phenotypes) A - B - AB - O
Genotypes:
•AA, Ai - blood type A
•BB, Bi - blood type B
•AB - blood type AB
•ii - blood type O
Ai x Bi ?
Sex Linkage
Most animal species have a special pair of chromosomes
that determine gender. In humans 22 pairs of autosomes
and 1 pair of sex chromosomes
Two types - X (large and has genes that control non gender related traits)
Y (small and only promotes the formation of male genitalia)
XX = female
XX x
XY = male
XY
Duchene’s Muscular Dystrophy
Color Blindness
Hemophilia
X
Y
X
XX
XY
X
XX
XY
Sex linked traits are traits
controlled by genes on the X
chromosome. Females have two,
males have only one. Females can
carry but not show defective sex
linked traits. Males CAN NOT be
carriers.
Other Non Mendelian Hereditary Conditions
Non-Disjunction:
Abnormal separation of chromosomes during meiosis. Gametes are formed having
too many (too much information) or too few chromosomes (not enough information)
and therefore unexpected characteristics arise. There are several human disorders
associated with this circumstance many of them have been described by the
CytoGenetic Gallery.
Some human disorders caused by non-disjunction include:
•Down Syndrome (three of the #21 chromosome)
•Turner Syndrome (X_ - only one sex chromosome - always the X chromosome)
•Klinefelter's Syndrome (XXY)
•Trisomy X (XXX)
Epistasis
Effects of one gene override or mask the phenotype of a second gene. Epistasis is not
dominance. Dominance only involves one gene which has two alleles (one dominant and
the other recessive). Epistasis involves two different genes where one gene (having two
alleles) impacts the expression of a different gene (also having two alleles).
Albinism - Melanin Production vs. Amount of melanin produced - two separate genes
directly impacting the color of skin
Pleiotropy
Pleiotropy occurs when a gene has more than one phenotypic expression. In a species of
mouse a gene that controls fur color has two alleles. Y and y. The heterozygous genotype
produces yellow fur. The hmozygous recessive produces gray fur. The homozygous dominant
creates a lethal condition that terminates development. Therefore a single gene controls fur
color and a critical stage during the embryonic development of the mouse.
Gene Imprinting
For imprinted genes, the gene copy that is turned on depends only on whether it came from the
mother or father, rather than on the classic laws of Mendelian genetics, where genes are either
dominant or recessive. It seems that certain genes are only functional with one active copy, not
zero and not two. A gene is made inactive by adding a methyl groups that blocks access to
RNA transcriptase and therefore makes the gene unable to be expressed. Fragile X Syndrome
is a disorder that appears to be a consequence of gene imprinting.
Extracellular Genes
Not all DNA is associated with chromosomes in the nucleus of Eukaryotic cells. The
mitochondria and plastids in plant cells have their own DNA which consists of small rings.
This DNA is responsible for the production of proteins essential to the organization and
function of their internal membranes (electron transport chain, ATP synthase).In humans the
mitochondria in every cell of an individual originated from the egg. The sperm only
contributes nuclear material.All mitochondrial DNA has maternal origin and this DNA may
impact the expression of nuclear genes.