NON MENDELIAN INHERITANCE PART III
Lethal Genes French geneticist Lucien Cuenot, experimentaly crosses on coat colour in mice, found a gene that was not consistent with mendelian predictions. Observations, Two yellow mice yielded approximately a 2:1 ratio of yellow to wild type, rather than the expected 3:1. Crosses yellow individuals to the recessive wild type, all yellow mice produced wild type progeny.
Conclusions Yellow mice were heterozygotes No yellow homozygotes were produced. Homozygosity for yellow is lethal, and that individuals died in utero When a genetic defect causes 100% mortality, it is termed a lethal gene.
The lack of a tail in the Manx cat is another trait caused by an allele that has a dominant effect in heterozygotes and is a lethal in homozygotes Manx allele M Normal allele m Cross between two Manx cat??
PLEIOTROPY Many of these genes may also have secondary or related effects. Ex: 01; Yellow coat colour in mice is an allele that affects more than one character; Body colour Heterozygotes Lethal Homozygotes Ex : 02; Mendel Flower colours violet and white also influenced seed colour The phenomenon in which a single gene affects two or more characteristics is called pleiotropy
Pleiotropic effects in humans Phenylketonuria is a genetic disorder where the body s enzyme, phenylalanine hydroxylase (PAH), is missing or malfunctioning so that it cannot properly break down the amino acid, phenylalanine (phe).
Phe is found in food containing proteins, but without PAH, consumption of such foods would lead up to a build-up of phe. The high level of phe affects the nervous system and other physical features.
Symptoms Mental Retardation Microcephaly (small head size) Lower IQ Stunted growth Musty body odor from the excess phenylalanline Fair skin, hair, and eyes (phenylalanine is linked to melanin production)
Penetrance and Expressivity All of the genes we have considered to this point have a definite genotype-phenotype relationship. However, some genes, a given genotype may or may not show a given phenotype. It is described as penetrance of a gene The level of penetrance can be calculated as the proportion of individuals with a given genotype who exhibit a particular phenotype Eg: When all individuals of a particular genotype have the same phenotype, the gene shows complete penetrance and the level of penetrance is 1.0.
Expressivity A particular genotype exhibits the expected phenotype, the level of expression or expressivity may vary. Ex:- A gene causes a detectable disease in most individuals with a given genotype some may be much more severely affected than others.
Fig. 13.18, Penetrance and expressivity
Causes of incomplete penetrance and variable expressivity? Environmental factors and other genes are known to influence the penetrance or expressivity of a gene. Environmental factors Eg: primrose have red flowers when grown at 24 o C but white flowers when grown above 32 o C. Temperature can affect coat coloration. Siamese cat fur in the extremities is darker due to cooler temperatures. The enzyme making darker pigment doesn t work well at the higher temperatures in the rest of the body.
Sex Linked Genes The Xs and Ys of Genetics
Chromosomes We have 2 types of Chromosomes: 1. Autosomal chromosomes pairs 1 through 22 in humans (non-sex chromosomes) 2. Sex chromosomes xx or xy What about a fruit fly with a 2n = 8?
Sex Linked Genes These chromosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome (X or Y) are said to be SEX LINKED
Males Men normally have an X and a Y combination of sex chromosomes. Since only men inherit the Y chromosome, they are the only ones that inherit Y-linked traits.
Females Females have 2 X chromosomes Designated as XX
X Linked Traits Men and Women can get the X- linked traits since both inherit X chromosomes.
SEX DETERMINATION XX = female Xy = male
Who decides? X X Mom can give X X X X X X Dad can give X or y y X y X y Dads determine sex of babies. If dad gives X with mom s X = girl If dad give y with mom s X = boy
SEX CHROMOSOMES CAN CARRY OTHER GENES TOO = SEX LINKED TRAITS
Sex chromosomes can carry other genes X-LINKED GENES: Genes carried on the X chromosome EX: Hemophilia Colorblindness Muscular Dystrophy
Hemophilia An X-linked genetically inherited recessive disease Can not clot their blood Severe bleeding from minor cuts Mostly in males
Make a cross with an X-linked gene Hemophilia is an X-linked recessive disease H Use for normal dominant blood h clotting gene. Use for recessive hemophilia gene. X h On X chromosome so write it as
Mother Without hemophilia = X X Without hemophilia = X X H H h H CARRIER With hemophilia = X X h h Father Without hemophilia = X y With hemophilia = X y h H
Make a cross with an X-linked gene Carrier Mom X X H X H X H X H y X H y Normal dad GIRLS 1/2 = normal 1/2 = look normal but are CARRIERS X h X H X h X h y BOYS 1/2 = normal 1/2 = hemophilia
Red-Green Color Blindness X-linked Cannot perceive red and green in the same Most often affect by males
Are you colorblind? Normal Color Vision: A: 29, B: 45, C: --, D: 26 Red-Green Color- Blind: A: 70, B: --, C: 5, D: -- 3. Red Color-blind: A: 70, B: --, C: 5, D: 6 4. Green Color-Blind: A: 70, B: --, C: 5, D: 2
Make a cross with an X-linked gene Colorblindness is an X-linked recessive disease B Use for normal dominant color gene b Use for recessive colorblind gene. X b On X chromosome so write it as
Mother B normal vision = X X normal vision = X X B b B CARRIER colorblind = X X b b Father normal vision = X y b colorblind = X y B
Color blindness is sex linked HOMOZYGOUS X B X b X B X b X B y y Normal Mom X colorblind dad X B X B X b X B y GIRLS = 100% carriers 100% normal BOYS =
X linked recessive genes appear in males than females. Females can be for X linked recessive traits. NEVER BE CARRIERS more often Males can carriers for X linked recessive genes. The either have trait OR are normal.
Muscular Dystrophy (MD) Characterized by a gradual irreversible wasting away of skeletal muscle. An X-linked trait most often passed on to sons from their mothers. Begins to weaken the legs of boys by age 3 and gets worse every year. No cure and inflicted usually die by age 30.
Y-linked Traits The Y chromosome is small and therefore does not contain many genes Y linked diseases are very rare Only passed from farther to son
Sex chromosomes can carry other genes Y-LINKED GENES: Genes carried on Y chromosome EX: Hairy ears genes Y linked only show up in males.
Make a cross with a y-linked gene Hairy ears is a Y linked dominant trait H Use for hairy ears. h Use for recessive normal ears. y H On y chromosome so write it as
Make a cross with a y-linked gene X X X X X X X ALL GIRLS = Normal ears y H X y H X y H ALL BOYS = Hairy ears