chapter 15 the chromosomal basis of inheritance l.
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Chapter 15: The Chromosomal Basis of Inheritance. Important Point:. If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!. Chromosomal Basis of Inheritance. Independent Assortment.

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Chapter 15: The Chromosomal Basis of Inheritance

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important point
Important Point:

If you are having trouble

understanding lecture material:

Try reading your text

before attending lectures.

And take the time to read it well!

independent assortment5
Independent Assortment

“Notice… that one-half of the offspring are expected to inherit a phenotype that matches one of he parental types.” p. 278, Campbell & Reece (2005)

2 locus test cross

Parental Types

Special dihybrid with b+ and vg+ on same chro-mosome




Parental Types

Recombinant Types

2-Locus Test Cross

“Expected” assumes Independent Assortment


Parental types

Basis of recombinant types

frequency of recombination
Frequency of Recombination

Complete linkage give frequency of recombination of 0%

Frequency of recombination

Note that max = 50% (which occurs when loci are either on separate chromosomes or very far apart on the same chromosome)

Note that it is the recombinant types that there are fewer or

Recombinant types

constructing genetic maps
Constructing Genetic Maps

Linkage means that two loci are found on the same chromosome

Ordering loci on chromosomes

Best way to interpret this data is that b and vg are about twice as far apart as b is from cn or cn is from vg. Hence, cn must be in the middle

linkage maps

>50% frequencies of recombination are produced by adding together smaller frequencies of recombination

Linkage Maps

Know what a linkage map is

sex linkage recessives in mammals
Sex-Linkage Recessives (in mammals)

e.g., some forms of hemophilia

e.g., some forms of color blindness

e.g., Duschenne muscular distrophy

Male genotype 100% controlled by what is in the egg

Differences 100% dependent on what is in the egg

female parent carrier
Female Parent Carrier

Males are more likely affected by recessive alleles, females by dominant alleles

both female carrier male affected
Both (female carrier & male affected)

Males express the genotype regardless of whether it is dominant or recessive

sex linkage inheritance patterns
Sex-Linkage Inheritance Patterns

Dominant or Recessive Sex-Linked Inheritance?

barr bodies dosage compensation
Barr Bodies & Dosage Compensation

One X chromosome is inactivated, to form what is known as a Barr body; this assures that both men and women have the same number of active X chromosome genes

X chromosomes are inactivated randomly and stay inactivated

Female mammals are mosaics

common aneuploidies

Polyploidies (e.g., in plants) are actually less serious than one too many (or one too few) autosomes (is a duplicate of all chromosomes)

  • Most abnormalities in numbers of autosomes, anueploidies(one too many or too few chromosomes), are very serious or fatal due to gene dosage imbalances
  • The fewer genes unbalanced, the less serious the condition; a trisomy is one extra chromosome
  • Down’s syndrome: Caused by a trisomy of chromosome number 21 (1 in 700 births)—mental retardation, mongoloid features, and heart defects
  • Most abnormalities of sex chromosomes do not affect survival
  • Klinefelter syndrome: Males with an extra X chromosome (XXY) (1 in 1000 male births)
  • Turner syndrome: Females missing one X chromosome (XO) (1 in 2500 female births)
Common Aneuploidies
down syndrome
Down Syndrome

Remember this

Remember this

xo turner syndrome
XO – Turner Syndrome

Remember this

An example of a monosomy

Remember this

Turner Syndrome (XO), Incidence: 1 in 2500 female births

xxy klinefelter syndrome
XXY – Klinefelter Syndrome

Remember this

Remember this

Klinefelter Syndrome (XXY), Incidence: 1:1000 male births




  Translocations and deletions can result in surviving partial trisomies or monosomies (again, the more genes involved, the more severe the consequences)

Found in patients with chronic myelogenous leukemia (CML)

cytoplasmic inheritance
Cytoplasmic Inheritance

Also plastid mutation

Maternally transmitted