Upon completion of this unit you will be able to:
1. State Mendel's three laws of inheritance.
2. Distinguish between genotype
and phenotype.
3. Provide examples of homozygous
and heterozygous genotypes.
4. Define and describe a gene and
homologous chromosomes.
5. Define and describe alleles.
6. Use the Punnet Spuare as a tool
in solving genetic problems.
7. Predict the probability of outcomes
for various genetic crosses.
8. Discuss the need to be able
to deal with probability vs. possibility
vs. reality when dealing
with the outcomes of genetic crosses.
9. Obtain the possible gametes
from any given genotype.
10. Use proper symbols for genes
and generations to illustrate crosses
with one pair of alleles
(monohybrid) and two pairs of alleles (dihybrid).
11. Determine the genotype and
phenotype ratios of the F1 and F2 generations
when given the genotype
of the parents.
12. Set up a back cross (test cross)
to determine the genetic make-up of
an individual.
13. Explain how crossing-over occurs
and what results from it.
14. Explain why genes located on the
same chromosome tend to be inherited together.
15. Differentiate between autosomes
and sex chromosomes.
16. Explain chromosomal determination
of sex in man.
17. Define nondisjunction and explain
its relevance to chromosomal genetics
(repeat objective from unit 7).
18. Compare the gene composition of
the X and Y chromosomes.
19. Work genetic problems dealing
with multiple alleles, polygenic inheritance
and X-linked characteristics.
20. Cite examples of incomplete
dominance.
21. Contrast sex-linked and sex-influenced
genes.
22. Explain the significance of lethal
genes.
23. Give examples which tend to illustrate
that the environment influences the phenotypic
development of the genotype.
24. Explain why one inherits the
potential for characteristics rather than the
characteristics per se.
25. Explain why many genetic studies
utilize Drosophila melanogaster.
26. Explain the importance of statistical
tests regarding genetic studies.
Unit References:
Text Chapters 10
http://www.mhhe.com/enger12
Unit 8: Patterns of Inheritance
When Dealing with Genetics you must deal with:
(Objective #8)
Possibility
vs. Probability vs. Reality
Gregor Mendel: Father of Modern Day
Genetics, Austrian Monk (1860)
Modern day modifications to what we
learned from Mendels thinking:
Traits
controlled by genes passed from generation to generation
Traits
expressed by 2 genes, one maternal, one paternal
1. Meiosis creates gametes,
one chromosome of each pair passed
2. Upon fertilization, each
parent contributes matching chromosomes (Homologous)
3. Chance alone determines
which chromosome goes to which gamete.
4. Dominant (default) gene
-----> codes for protein (symbol = capital letter)
Recessive gene ---->
does not transcribe (symbol = lower case letter)
Additional Vocabulary:
(Objective #2, 3, 5) text pg 192-193
Genotype-
Phenotype-
Genome-
Homozygous (dominant or recessive)-
Heterozygous-
Alleles-
Mendel's Laws of Heredity:
(Objective #1) text pg. 196-198
1. Law of Dominance:
2. Law of Segregation:
3. Law of Independent Assortment:
Check Out:
Talking Glossary of Genetic Terms
Mendel Web
SOLVING GENETICS PROBLEMS:
(Objective #6, 7, 8, 9, 10, 11)
text pgs 198- 203
Probability vs. Possibility
vs. Reality
Predict: Offspring phenotypes, genotypes and ratios represented
Single Factor Cross: Monohybrid
5 step process: pg 198-201
Earlobes: Free vs. Attached
Step 1: Assign a symbol for
each allele
E = free
e = attached
Step 2: Determine genotypes of parents
and indicate a mating
2 Heterozygous parents
Ee X Ee
(could have been EE X EE; EE X Ee etc.)
Step 3: Determine all possible
gametes each parent can produce
Ee can produce (or
pass to offspring) gametes containing E or e.
***Use Punnett Square to show
mating***
Each box
represents
a possible
offspring
Step 4: Determine all gene combinations
resulting when these gametes unite
(fill in Punnett)
Step 5: Determine Phenotype of each gene combination
EE = Free Ee, eE = Free ee = attached
*Genotype ratio 1:2:1
Phenotype ratio 3:1
(Objective #6 - 11)
Mate: Homozygous Dom Tall (T) X
Homozygous Rec Short (t)
Parents ??? (Considered
to be the P generation)
Gametes ???
Offspring ??? (Considered
to be the F1 generation)
Mate: Two F1 Individuals of
previous cross (heterozygous tall)
Mating ???
Gametes ???
Offspring ??? Ratios
???
Problem #3 of problem sheet details similar
crosses and will be very similar to
Monohybrid test question
(Objective #6-11)
Monohybrid examples
(text pg 200): do genotype and phenotype ratios change according
to cross
In humans: N is the gene for
normal phenyalanine metabolism
n is the gene
possessed by PKU individuals
What ratios result from:
Homozygous normal X heterozygous
(carrier) normal
Heterozygous normal X homozygous PKU
Check Out:
Punnett Square Animations
(Objective #6-11)
Double Factor Cross:
Dihybrid Inheritance- inheritance patterns
when two traits are
involved Text pgs 201-202
Alleles: T - Tall t - short R - Rough r- smooth
1. Cross homozygous Tall/Rough
with homozygous short/smooth
Parents??
Gametes??
F1??
2. Cross two F1 offspring
(both heterozygous for both traits)
Mating??
Gametes??
F2??
Problem: Since homozygous dominant and heterozygous
individuals appear the
same how do we tell them apart
genetically? (Ex. TT and Tt both appear tall;
TTRR and TtRr both appear to
be Tall and Rough)
Test Cross (back cross) Somewhat old school
(Objective #12)- crosses used to preserve
pure lines;
identify heterozygotes
*Cross questionable dominant
with a HOMOZYGOUS RECESSIVE individual
*Monohybrid - tall plant, is it TT or Tt?
if TT then TT X tt
will only produce Tt (tall) offspring
if Tt then Tt X tt will give Tt (tall) or tt (short) 1:1 ratio
*Dihybrid - Tall/ Rough, is it TTRR or TtRr?
(or TTRr; TtRR)
if TTRR then TTRR X ttrr
will only produce TtRr (Tall/ Rough) offspring
if TtRr then TtRr X ttrr will produce TtRr; Ttrr, ttRr, ttrr in 1:1:1:1 ratio
Check Out:
Biology Project- Dihybrid Cross Problems
(Objective #13
) Text pg 182
CROSSING OVER- swap of homologous genetic
info; occurs during meiosis,
creates genetic variability
Check Out:
Crossingover of Chromosomes during Meiosis
Assortment of Chromosomes in Meiosis
Homologous chromosomes (no crossover)
Homologous chromosomes (single crossover)
chiasma
Alert!! Trivial info coming pencils could explode if attempts are made to copy all of this!
*Crossing over contributes to variability
of genome
Total possible combos for
one individual = 2n (2 possible outcomes; n = total # of
chromosome pairs; humans
n = 23)
Example, one coin two possibilities
21 = 2 (head or tail)
two coins two possibilities
for each coin 22 = 4 possible outcomes
Humans 2 23 = 8,388,608 possible combos (remember the encyclopedias ?)
if one crossover (there are
usually many)
423 =
70,368,744,000,000 possible combos for one individual
When fertilization takes place: with one crossover (4 23 )2 or
4,951,760,200,000,000,000,000,000,000
different possible offspring for each
couple!
Chromosome Theory (Objective
#14, 15, 16)- Chromosomes carry hereditary units
(genes), since traits are more
numerous than chromosomes, then each chromosome
carries many traits.
-Gene linkage and linkage groups. Text pg 208-210
Humans: 23 pairs of chromosomes, therefore, 23 linkage groups
22 pairs of autosomes (not
involved in sex determination)
1 pair of sex chromosomes
(alone determine sex of individual), X and Y
Text pg. 597-599
XX = female;
XY = male
Upon meiosis: female gametes
can only contain an X
male gametes
can contain an X or a Y (50:50 prob of either)
Male determines sex of offspring (did Henry the VIII know?!?!)
Potential Problems with Meiosis:
(Objective #17) text pg 185-187, 597-599
Nondisjunction- failure of
chromosomes to properly divide during meiosis
In Sex Chromosomes can be tolerated:
Turners = XO; Kleinfelters = XXY
***XYY male
In Autosomes, nondisjunction is rarely tolerated: Down's syndrome
Check Out:
Introduction to Chromosome Abnormalities
National Down's Syndrome Society
A Guide For XXY Males and Their Families
REAL WORLD GENETICS: GENE INTERACTION
1. Incomplete Dominance (lack of dominance);
Codominance (Objective #20)
text
pg 202- 204
-two unlike alleles both expressed
equally, appear to produce intermediate
phenotype in the heterozygote
Snap dragon petal color (roan cattle,
4 O'clocks)
CW- white flower;
CR- red flower (alleles at same locus)
C W CW X CR CR -------> CWCR (all Pink)
CWCR X CWCR -------> CWCW : C WCR : CR CR (in 1:2:1 ratio)
2. Multiple Alleles
(Objective #19) text pg 205-
characteristics determined by
more than two different alleles.
(Multiple alternative forms
of a gene) Ex. Cystic Fibrosis, 330 diff forms of that gene.
Human Blood Types: A, B, AB, o
Alleles: A, B (are codominant)
and o (A and B are dominant to o)
to be type o you
must be homozygous recessive
Type AB mother says Type o man is father of her Type A child. Possible?
Type AB mother says Type o man is father of her Type AB child. Possible?
Check Out:
Genes and Blood Type
ABO Genetics
3. Polygenic inheritance
(Objective #19) text pg 206-
("Mailman", "Milkman",
"UPS man", "Schwan man" Syndrome)
-inheritance determined by
several genes at different locations. Ex. Eye color, skin
color, height.
*Results in wide variation in the
expression of the trait, statistically will fit a bell shaped
curve.
Skin color: AABBCC (Black) X aabbcc (White) -----> AaBbCc (Mullato)
AaBbCc ---> 3 different genes each with 2 possible outcomes = 8 combos
Check Out:
Polygenic Inheritance
Poly Gene Inheritance
4. Pleiotropy
5. Sex Linked Inheritance
(Objective #18, 19, 21) text pg 208-210
- refers
to inheritance of genes
on the X chromosome.
-since females have XX and
males XY then all the genes on the X for the male are
expressed whether they are
dominant or recessive
X chromosome
Y chromosome
Long
short
many genes, many alleles
few alleles (perhaps a few in common with X)
Y contains male determining genes
**X and Y are not homologous
Red/ Green Colorblindness: X N = normal; Xn = colorblind; Y = male, contains no cb genes
Cross: Normal male (XN
Y) X heterozygous female (normal carrier; XNXn
)
*Did son inherit colorblindness from father?
*What do sons inherit from their fathers?
Calico Cats are females: XB = black; XG = gold and are sex linked
Check Out:
NOVA Sex Determination
6. Sex- influenced
(Objective #21)- Determined by autosomes (not X or Y).
Phenotype
appears in both sexes but different
genotypes are necessary in males and females to
produce phenotype
Ex. pattern baldness; Index finger longer than or equal to 4th finger dom in females
**Hormonal differences between sexes usually responsible
7. Lethal genes
(Objective #22)- mutations to genes that lead directly to
the death of
organism before reproductive
age ex. Huntingtons disease (rare in that it appears
after reproductive age)
INHERITANCE CONDITIONS
Nature vs. Nurture
(Objective #23, 24) pg 211- 213
debate but no clear answer,
wise person agrees influence is from both
-possible for two identical
genotypes (twins) to differ in phenotypes
Environment effects expression-
even though you inherit a trait, the environment may
not allow you to express
it
Internal environment (chemical)-
Ex. male voice, genes for pitch; pre puberty
vs. post puberty
External environment-
Ex. Snowhare rabbit, siamese cat
Coat color genes
may not reveal themselves unless the temp of skin is
above/below
a certain point, *most dramatically seen on feet, ears
Humans- freckles appear upon exposure to sun
Check Out:
Your Genes, Your Health
National Society of Genetic Counselors