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Bio Chapter 14 Notes
Mendelian and Human Genetics
Inheritance Patterns:
A) Complete Dominance
•

Complete Dominance- When dominant homozygous and heterozygous exhibit
dominant phenotype and homozygous recessive exhibits recessive phenotype

•

Genotype- An organism’s DNA or genes
o Shown through alleles

•

Phenotype- Expressed trait

•

Dominant allele is a capitol, recessive in a lower case (A,a)

•

Example
o Alleles A and a have a complete dominance inheritance patter
§

Dominant trait phenotype comes from genotypes AA or Aa

§

Recessive trait phenotype comes from genotype aa

B) Incomplete Dominance
•

Incomplete Dominance- When heterozygous individuals exhibit an intermediate
phenotype; not really blending, simply intermediate

•

Example
o Allele R is a red snapdragon and r is a white snapdragon and have
incompletely inheritance pattern
§

Dominant (red) trait phenotype comes from genotype RR

§

Recessive (white) trait phenotype comes from genotype rr

§

The other trait phenotype (pink) comes from the genotype Rr

C) Codominance
•

Codominance- When heterozygous individuals express both alleles in the
organism; for which two or more possible alleles each affect the genotype in
separate yet distinguishable ways

•

Examples
o Blood types
§

For blood type phenotypic designations MN, AB, AO, and BO,
represent individuals that have heterozygous genotypes because
both alleles are being expressed

o Animal colorations
§

Oreo cows, snake patterns, cat with two sides of the face colored
different

D) Complete Dominance Continued
•

Dominant traits are not always more common than recessive traits for a specific
genetic characteristic
o Commonality depends on frequency of the allele types
o Example
§

If the frequency of A is 10% and a is 90%, then the recessive trait
will be more common

§

P = polydactyl condition (6th digit on hand/foot) and p = normal
•

Although polydactyl is dominant, it is FAR less common
than normal

•

Fun fact- 1 in 400 babies are born with extra digits in US
(higher frequency in Amish community)

•

Dominance- When a dominant allele masks the expression of the recessive
alleles in individuals with a heterozygous phenotype
o Only applies with complete dominance inheritance pattern
o Do NOT really discuss dominant alleles when discussing either
incomplete dominance or codominance

Principles / Laws in Genetics:
A) Law of Dominance
•

Law of Dominance- The law of the dominant allele to mask the expression of the
recessive allele in the heterozygous individuals
o The phenotype of the individual will be the dominant phenotype

•

Relevant to:
o Meiosis only
o Both monohybrid and dihybrid crosses
o Complete dominance only

B) Law of Segregation
•

Law of Segregation- Separation of homologous chromosomes
o Homologous chromosomes refer to those similar chromosomes with one
from each parent
o They separate during anaphase of meiosis 1 because meiosis eventually
gives rise to haploid egg and sperm

§

Egg and sperm must have one allele for each character

o Think egg vs sperm when creating Punnett squares and writing alleles
outside of the squares
•

Relevant to:
o Meiosis only
o Both monohybrid and dihybrid crosses
o All three types of inheritance patterns
§

Complete dominance, incomplete dominance, and co-dominance

C) Law of Independent Assortment
•

Independent Assortment- Homologous chromosomes of a particular number
(chromosome 1 from each parent) separate in a manner that does not influence the
separation of homologous chromosomes of a different number
o

Always of different chromosomes

o Any egg or sperm could have any of the four combinations of
chromosomes 1 and 2:
§

Chromosome 1

Chromosome 2

•

Mom

•

Mom

Dad

•

Dad

Dad

•
•

Mom

Dad

Mom

Relevant to:
o Meiosis only

o Dihybrid crosses only because talking about different chromosomes
o All three types of inheritance patterns
§

Complete dominance, incomplete dominance, and co-dominance\

Using Punnett Squares to Complete Genetic Crosses:
1
...
) Given a monohybrid cross with incomplete dominance between RR and rr with R=red
flower and r=white flower

•

F1 generation’s genotype and phenotype
o Genotype- 100% heterozygous Rr
o Phenotype- 100% pink

•

F2 generation’s genotype and phenotype (fill in Punnett squares in notes and
check them)
o Genotype- 1 RR : 2 Rr : 1 rr
o Phenotype- 1 red : 2 pink : 1 white

3
...
) Given a dihybrid cross with complete dominance between YYRR and yyrr with
Y=yellow, y=green, R=round, and r=wrinkled
•

F1 generation’s genotype and phenotype
o Genotype- 100% YyRr
o Phenotype- 100% yellow and round

•

F2 generation’s genotype and phenotype
o Genotype- 9 Y-anything R-anything : 3 Y-anything rr: 3 yy R-anything: 1
yyrr

o Phenotype- 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green
wrinkled
•

Classic dihybrid ratio of complete dominance inheritance pattern starting with two
true breeding
o Genotype- 9 dominant dominant : 3 dominant recessive : 3 recessive
dominant : 1 recessive recessive
o Phenotype- 9 dominant dominant : 3 dominant recessive : 3 recessive
dominant : 1 recessive recessive

5
...
) Given a cross between two blood type A individuals and co-dominance inheritance
pattern between ABO and given that the phenotype A blood can either have AA or AO
genotype, what are the possible genotypes and phenotypes of offspring from two A blood
individuals?
•

AA and AA
o Genotypes- 100% AA
o Phenotypes- 100% A blood type

•

AA and AO
o Genotypes- 2 AA : 2 AO
o Phenotypes- 100% A blood type

•

AO and AO
o Genotypes- 1 AA : 2 AO : 1 OO
o Phenotypes- 3 A blood types : 1 O blood type

7

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