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Chromosomes and Inheritance

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1 Chromosomes and Inheritance
Chap 15 AP Bio Chromosomes and Inheritance

2 Locating Genes on Chromosomes
Are located on chromosomes Can be visualized using certain techniques Figure 15.1 Karyotype

3 Scientist Discoveries
Morgan worked with fruit flies Because they breed at a high rate A new generation can be bred every two weeks They have only four pairs of chromosomes

4 Sex Linked Trait (White Eyes)
Morgan first observed and noted Wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type Are called mutant phenotypes Figure 15.3

5 Fly Experiment In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes

6 Males more often have sex linked traits
Sex Linked White Eyes Figure 15.4 The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F1 red-eyed female to an F1 red-eyed male to produce the F2 generation. RESULTS P Generation F1 X F2 Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes. EXPERIMENT

7 Morgan determined that
Genes that are close together on the same chromosome are linked and do not assort independently Unlinked genes are either on separate chromosomes of are far apart on the same chromosome and assort independently Parents in testcross b+ vg+ b vg b+ vg+ b vg b vg Most offspring X or

8 A linkage map Is the actual genetic map of a chromosome based on recombination frequencies Recombination frequencies 9% 9.5% 17% b cn vg Chromosome The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossover would “cancel out” the first and thus reduce the observed b–vg recombination frequency. In this example, the observed recombination frequencies between three Drosophila gene pairs (b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between the other two genes: RESULTS A linkage map shows the relative locations of genes along a chromosome. APPLICATION TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between two genetic loci is proportional to the distance separating the loci. The recombination frequencies used to construct a linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depicted in Figure The distances between genes are expressed as map units (centimorgans), with one map unit equivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data. Figure 15.7

9 Crossing over The farther apart genes are on a chromosome
The more likely they are to be separated during crossing over The frequency of cross over between two linked genes is proportional to the distance between them. Linkage Maps are a genetic map based on recombination frequencies. Linked genes are on the same chromosome. New combinations of linked genes are due to crossing over.

10 Sex Determination In humans and other mammals
There are two varieties of sex chromosomes, X and Y Figure 15.9a (a) The X-Y system 44 + XY XX Parents 22 + X Y Sperm Ova Zygotes (offspring)

11 Sex Chromosomes The sex chromosomes
Have genes for many characters unrelated to sex A gene located on either sex chromosome Is called a sex-linked gene SRY gene is present on the Y chromosome and triggers male development.

12 Different systems of sex determination Are found in other organisms
Figure 15.9b–d 22 + XX X 76 + ZZ ZW 16 (Haploid) (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system

13 Sex Linked disorders Some recessive alleles found on the X chromosome in humans cause certain types of disorders Color blindness Duchenne muscular dystrophy Hemophilia Red-Green colorblindness is a sex-linked recessive trait.

14 X inactivation In mammalian females
One of the two X chromosomes in each cell is randomly inactivated during embryonic development and remains as a dark compact body in the nucleus of female somatic cells. This is called a Barr Body

15 Female X inactivation If a female is heterozygous for a particular gene located on the X chromosome She will be a mosaic for that character Two cell populations in adult cat: Active X Orange fur Inactive X Early embryo: X chromosomes Allele for black fur Cell division and X chromosome inactivation Black Figure 15.11

16 Barr Bodies in Female somatic cells
Female Genotype?

17 Chromosomal Alterations
Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders Large-scale chromosomal alterations Often lead to spontaneous abortions or cause a variety of developmental disorders

18 Nondisjunction of sister chromatids in meiosis II
Pairs of homologous chromosomes do not separate normally during meiosis Gametes contain two copies or no copies of a particular chromosome Figure 15.12a, b Meiosis I Nondisjunction Meiosis II Gametes n + 1 n  1 n – 1 n –1 n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b)

19 Aneuploidy Results from the fertilization of gametes in which nondisjunction occurred Is a condition in which offspring have an abnormal number of a particular chromosome

20 Results of Nondisjunction
If a zygote is trisomic It has three copies of a particular chromosome If a zygote is monosomic It has only one copy of a particular chromosome

21 Polyploidy Is a condition in which there are more than two complete sets of chromosomes in an organism

22 Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion Duplication Inversion Translocation

23 Chromosome mutations Figure 15.14a–d A B C D E F G H Deletion
Duplication M N O P Q R Inversion Reciprocal translocation (a) A deletion removes a chromosomal segment. (b) A duplication repeats a segment. (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment from one chromosome to another, nonhomologous one. In a reciprocal   translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return.

24 Down syndrome Is usually the result of an extra chromosome 21, trisomy 21 Figure 15.15

25 Aneuploidy of Sex Chromosomes
Klinefelter syndrome Is the result of an extra chromosome in a male, producing XXY individuals Turner syndrome Is the result of monosomy X, producing an X0 karyotype

26 Certain cancers Are caused by translocations of chromosomes
Cri du chat Is a disorder caused by a deletion in a chromosome Figure 15.16 Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22

27 Genetic Disorders Phenylketonuria PKU- autosomal recessive, can be controlled by regulating diet Huntington’s- dominant single allele, does not appear until 35-45yrs of age. If one of your parents is diagnosed what is the probability you will have it? Tay-Sachs disease (autosomal recessive)- can’t break down lipids and this affects brain development Sickle Cell Anemia (autosomal recessive)- caused by a substitution of the wrong amino acid Cystic Fibrosis- defect in the membrane proteins that normally function in Chloride ion transport

28 A pedigree Is a family tree that describes the interrelationships of parents and children across generations

29 Inheritance patterns of particular traits Can be traced and described using pedigrees
Ww ww WW or First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ff FF or Ff FF Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

30 Pedigrees Pedigree a diagram of a family tree showing the occurrence of heritable characters in parents offspring over multiple generations. Can also be used to make predictions about future offspring

31 Karyotype- used to identify disorders

32 Genetic testing Pedigrees, Karotypes, amniocentesis, and chronic villi sampling are all ways to test for genetic disorders.

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