Nov 1 Chapter 8 (continued)

 

Meiosis and crossing over (continued)

 

8)     Crossing over (Fig 8.18A, p144)

- The exchange of segments between of homologous chromosomes.

- Occurs during synapsis (prophase I).

- Results in genetic recombination, which contributes to genetic variation.

 

 

 

Chromosome abnormality

 

1)     Karyotype

- orderly display of chromosomes for diagnosis

- treat cells with chemicals to arrest mitosis at methaphase

- stained chromosomes are observed under a microscope

- chromosomes are sorted according to size and shape for identification

- allows identification of gender and chromosome abnormalities

 

 

 

2)     Abnormality in chromosome number

a. Nondisjunction (Fig 8.21, p148)

- Failure in chromosome separation during meiosis.

- Results in abnormal number of chromosomes in gamete, which in turn results in abnormal number of chromosomes in zygote

- Abnormal number of chromosomes normally results in spontaneous abortion (miscarriage)

 

b. Down syndrome

- Three chromosome 21 (trisomy 21, Fig 8.20A, p147).

- Most common chromosome abnormality (1/700)

- Altered facial features, various health problems, shorter life span.

- Incidence of Down syndrome increases with the age of the mother. (Fig 8.20C)

- Fetal testing is possible (will be discussed in Chapter 9).

 

c. Abnormal number of sex chromosomes

-Abnormal number of sex chromosomes does not always result in abnormal development.

XXX, XYY – apparently normal

XXY – Abnormal sex organ development (Klinefelter syndrome). 

Normal intelligence.

XO – (only one X chromosome) Abnormal  development (Turner syndrome). 

            Normal intelligence.

 

3)     Alteration of chromosome structure

deletion

duplication

inversion

translocation

May result in birth defects or cancer.

 

 

Chapter 9 Patterns of Inheritance

 

Mendel’s Principles

 

1)     Gregor Mendel’s experiments

1860s

- observed patterns of inheritance in pea plants

- controlled fertilization (Fig 9.2C, p157)

self-fertilize or cross-hybridize (cross)

- scored characteristics that were easy to observe

 

Genetic terminology

hybrids – offspring of two different varieties

P generation – parents

F1 generation – hybrid offspring

F2 generation – generation resulted from either self-fertilization of F1 generation or fertilization within F1 generation.

 

Fig 9.3A, p158

 

 

 

2)     Mendel’s principle of segregation

3)     Mendel’s principle of independent assortment