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Chapter 15: Chromosomal Basis of Inheritance

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1 Chapter 15: Chromosomal Basis of Inheritance

2 Sex linked traits 1910 | 1933 Genes are on sex chromosomes
as opposed to autosomal chromosomes first discovered by T.H. Morgan at Columbia U. Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male

3 Classes of chromosomes
autosomal chromosomes sex chromosomes

4 Discovery of sex linkage
true-breeding red-eye female true-breeding white-eye male X P 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

5 What’s up with Morgan’s flies?
x x RR rr Rr Rr r r R r R Rr Rr R RR Rr R Rr Rr r Rr rr 100% red eyes 3 red : 1 white

6 Genetics of Sex X Y X XX XY X XX XY
In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome develop as a male: XY no redundancy X Y X XX XY X XX XY 50% female : 50% male

7 Let’s reconsider Morgan’s flies…
x x XRXR XrY XRXr XRY Xr Y XR Y XR XR XRXr XRY XRXR XRY XR Xr XRXr XRY XRXr XrY 100% red females 50% red males; 50% white males 100% red eyes

8 Genes on sex chromosomes
Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones many effects = pleiotropy! X chromosome other genes/traits beyond sex determination mutations: hemophilia Duchenne muscular dystrophy color-blindness Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and loss of coordination.

9 Human X chromosome Sex-linked usually means “X-linked”
Duchenne muscular dystrophy Becker muscular dystrophy Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Adrenal hypoplasia Glycerol kinase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation PRPS-related gout Lowe syndrome Lesch-Nyhan syndrome HPRT-related gout Hunter syndrome Hemophilia B Hemophilia A G6PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked Androgen insensitivity Chronic granulomatous disease Retinitis pigmentosa-3 Norrie disease Retinitis pigmentosa-2 Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Albinism-deafness syndrome Fragile-X syndrome Immunodeficiency, X-linked, with hyper IgM Lymphoproliferative syndrome Ornithine transcarbamylase deficiency Human X chromosome Sex-linked usually means “X-linked” more than 60 diseases traced to genes on X chromosome

10 Map of Human Y chromosome?
< 30 genes on Y chromosome Sex-determining Region Y (SRY) linked Channel Flipping (FLP) Catching & Throwing (BLZ-1) Self confidence (BLZ-2) note: not linked to ability gene Devotion to sports (BUD-E) Addiction to death & destruction movies (SAW-2) Scratching (ITCH-E) Spitting (P2E) Inability to express affection over phone (ME-2) Selective hearing loss (HUH) Total lack of recall for dates (OOPS) Air guitar (RIF)

11 Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors.
These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein.

sex-linked recessive Hemophilia XHXh XHY Hh x HH XH XHXh XH Y male / sperm Xh XHXH XHXH XHY XHY XH Xh female / eggs XH XHY XHXh XHXh XhY XhY Y carrier disease

13 X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh
one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” patches of black XH XHXh Xh tricolor cats can only be female patches of orange

14 Male pattern baldness Sex influenced trait
autosomal trait influenced by sex hormones age effect as well = onset after 30 years old dominant in males & recessive in females B_ = bald in males; bb = bald in females

15 Environmental effects
Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences Color of Hydrangea flowers is influenced by soil pH

16 Changes in chromosome structure
deletion loss of a chromosomal segment duplication repeat a segment inversion reverses a segment translocation move segment from one chromosome to another error of replication error of crossing over

17 Chromosomal abnormalities
Incorrect number of chromosomes nondisjunction chromosomes don’t separate properly during meiosis breakage of chromosomes deletion duplication inversion translocation

18 Nondisjunction Problems with meiotic spindle cause errors in daughter cells homologous chromosomes do not separate properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes 2n n-1 n n+1

19 Alteration of chromosome number
error in Meiosis 1 error in Meiosis 2 all with incorrect number 1/2 with incorrect number

20 Nondisjunction Baby has wrong chromosome number trisomy monosomy
cells have 3 copies of a chromosome monosomy cells have only 1 copy of a chromosome n+1 n n-1 n trisomy 2n+1 monosomy 2n-1

21 Human chromosome disorders
High frequency in humans most embryos are spontaneously aborted alterations are too disastrous developmental problems result from biochemical imbalance imbalance in regulatory molecules? hormones? transcription factors? Certain conditions are tolerated upset the balance less = survivable but characteristic set of symptoms = syndrome

22 Down syndrome Trisomy 21 3 copies of chromosome 21 1 in 700 children born in U.S. Chromosome 21 is the smallest human chromosome but still severe effects Frequency of Down syndrome correlates with the age of the mother Trisomy 13 occurs in about 1 out of every 5,000 live births. It is a syndrome with multiple abnormalities, many of which are not compatible with life. More than 80% of children with trisomy 13 die in the first month. Trisomy 13 is associated with multiple abnormalities, including defects of the brain that lead to seizures, apnea, deafness, and eye abnormalities. The eyes are small with defects in the iris (coloboma ). Most infants have a cleft lip and cleft palate, and low-set ears. Congenital heart disease is present in approximately 80% of affected infants. Hernias and genital abnormalities are common. Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 3,000 live births, and affecting girls more than three times as often as boys. The presence of an extra number 18 chromosome leads to multiple abnormalities. Many of these abnormalities make it hard for infants to live longer than a few months. The cri du chat syndrome is caused by the deletion of information on chromosome 5. It is likely that multiple genes on chromosome 5 are deleted. One deleted gene, called TERT (telomerase reverse transcriptase) is involved in control of cell growth, and may play a role in how some of the features of cri cu chat develop. The cause of this rare chromosomal deletion is not known, but it is expected that the majority of cases are due to spontaneous loss of a piece of chromosome 5 during development of an egg or sperm. A minority of cases result from one parent carrying a rearrangement of chromosome 5 called a translocation. Between 1 in 20,000 and 1 in 50,000 babies are affected. This disease may account for up to 1% of individuals with severe mental retardation. Infants with cri du chat syndrome commonly have a distinctive cat-like cry. They also have an extensive grouping of abnormalities, with severe mental retardation being the most important.

23 Down syndrome & age of mother
Mother’s age Incidence of Down Syndrome Under 30 <1 in 1000 30 1 in 900 35 1 in 400 36 1 in 300 37 1 in 230 38 1 in 180 39 1 in 135 40 1 in 105 42 1 in 60 44 1 in 35 46 1 in 20 48 1 in 16 49 1 in 12 Rate of miscarriage due to amniocentesis: 1970s data 0.5%, or 1 in 200 pregnancies 2006 data <0.1%, or 1 in 1600 pregnancies

24 Genetic testing Amniocentesis in 2nd trimester Analysis of karyotype
sample of embryo cells stain & photograph chromosomes Analysis of karyotype

25 Sex chromosomes abnormalities
Human development more tolerant of wrong numbers in sex chromosome But produces a variety of distinct syndromes in humans XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female

26 Klinefelter’s syndrome
XXY male one in every 2000 live births have male sex organs, but are sterile feminine characteristics some breast development lack of facial hair tall normal intelligence

27 Klinefelter’s syndrome
How many Barr bodies would you expect?

28 Jacob’s syndrome male XYY Males 1 in 1000 live male births
extra Y chromosome slightly taller than average more active normal intelligence, slight learning disabilities delayed emotional maturity normal sexual development

29 Trisomy X XXX 1 in every 2000 live births produces healthy females
Why? Barr bodies all but one X chromosome is inactivated How many Barr bodies would you expect?

30 Turner syndrome Monosomy X or X0 1 in every 5000 births
varied degree of effects webbed neck short stature sterile How many Barr bodies would you expect?

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