Much ha happened since Mendel

Transcription

1 Chapter 15 Chromosomal Basis of Inheritance Much ha happened since Mendel We can show genes are located at particular loci on chromosomes Using fluorescent dye to mark a particular gene 1

2 The use of these improved techniques has reinforced Mendel's laws of inheritance It is truly the behavior of the chromosomes that accounts for segregation and independent assortment Recall that Mendelian genetics does not explain all situations The number of genes in a cell is far greater than the number of chromosomes How many genes in human genome?? At least 10,000 perhaps as many as 25,000 Some do not appear to follow Mendel s laws because that are linked genes located very near one another on a chromosome and tend to be inherited together 2

3 Sex linked genes also deviate from classical Mendelian laws Chromosomal basis for sex Phenotypical males and females in mammals differ due to the two varieties of sex chromosomes Designated X and Y XX female XY - male In human the anatomical signs of age begin to appear at about two months of development Prior, the gonads are essentially the same Differences in hormones will produce the development of the testes or ovaries 3

4 In addition to determining sex, the sex chromosomes have many genes for other characters, especially the X chromosome Genes located on either sex chromosome is known as sex-linked genes Do not confuse with linked genes which occur on the same chromosome! If a sex-linked trait is due to a recessive allele, a female express the phenotype only if she is a homozygote recessive. A male has only one locus (why -only one X chromosome) A male which receives a recessive allele from his mother will express the recessive phenotype Color-blindness Example of sex-linked recessive trait Males more likely to exhibit phenotype Hemophilia another example of sexlinked recessive disorder 4

5 X inactivation Although female mammals inherit two X chromosomes, one X chromosome becomes almost inactive during embryonic development The condensed X chromosome is known as a Barr body- most genes on the Barr body are not expressed This explains why tortoise shell and calico cats are always female!!! Calico cats actually have a gene for a third color thrown in here!!! 5

6 Changes in chromosome number or structure cause some genetic disorders Typically, plants can tolerate more large scale changes to chromosome structure and number than animals Large scale changes in chromosomes often leads to spontaneous abortions of fetus. Smaller scale changes can simply affect the phenotype Abnormal chromosome number Aneuploidy abnormal number of chromosomes If a cell has one extra chromosome (2n +1) it is said to be trisonomic If a cell has a chromosome missing (2n-1) it is said to be monosomic The cause of this is usually a mishap called nondisjunction Occurs during meiosis where parts of homologous chromosomes or sister chromatids do not separate as they should. 6

7 Human disorders due to aneuoploidy Down Syndrome (trisomy 21) Approx. 1 of 700 children in USA Trisonomic for 21 st pair Characteristic facial appearance Short stature Heart defects Mental retardation Poor sexual development Odds of this nondisjunction increase in mothers over the age of 30 7

8 Aneuploidy of sex chromosomes Klinefelter syndrome Extra X chromosome in males (XXY) Typically sterile male phenotype Occurs about 1 in 2000 births Turner syndrome Monosomic X (XO) Phenotypically female and sterile 8