Chapter 15- Chromosomes & Inheritance Jessica Liu and Cassandra Xia TJ Biology Olympiad
Wild type- natural type - Usually denoted by + Mutant phenotype
Drosophila Melanogaster - Common experimental organism - Four pairs of chromosomes
Sex linked genes- genes located on a sex chromosome. More specifically, the X-chromosome in an XY system animal because the Y-chromosome doesn’t carry many genes. - Notice that F2 still follows the 3:1 ratio of complete dominance - Notice that in this case, female cannot show mutant phenotype. What must be the genotype of the P generation in order to create one female with the mutant phenotype in the F2? Sex-linked disorders are rarer in females for this reason.
Figure 1. Morgan notices that the F2 generation shows 3:1 ratio, but all females had red eyes and it is half of the males that had white eyes. Morgan hypothesizes that the gene for eye color is on the X chromosome
But wait…. Sex linked genes are only a special case of linked genes… Linked genes are genes on the same chromosome. Important for the reasons below:
Genetic recombination- production of offspring with new combinations of traits inherited from two parents 1. by independent assortment of different chromosomes 2. by crossing over of linked genes
(parental types- offspring whose phenotype that matches one of the parents recombinants- offspring who have different combinations of a set of traits than either parent) Figure 2. If two genes are linked, you get uneven results like this.
The distribution is not what would be expected from drawing a Punnett square. This is because the genes for color and wing shape are on the same chromosome and thus do not undergo independent assortment.
#recombinants Recombination frequency = × 100% = number of map units apart on chromosome total _ offspring
Figure 3. This is a linkage map. Calculate recombination frequencies between pairs of genes to plot a linkage map.
genetic map- list of gene loci along a chromosome linkage map- genetic map based on recombination frequencies (genetic map drawn to scale) cytological map- map with respect to chromosomal features such as bands that can be seen by a microscope
Sex chromosome systems - XY system o Mammals, fruit flies, some insects - X-0 system o Grasshoppers, crickets, roaches, some insects o Females XX, males X0 (only one chromosome) - Z-W system o Birds, some fishes, some insects o Females ZW, males ZZ - Haplo-diploid system o Bees and ants o No sex chromsomes- females from fertilized ova and are thus diploid
Sex-linked genes—genes located on either sex chromosome (although usually the X-chromosome in humans) Females express recessive allele only if they are homozygous for the recessive allele. However, males have only one copy of the chromosome. So… they are hemizygous (homozygous and heterozygous mean the same thing) Disorders (X-linked) · Color blindness—characterized by inability to see one or more of the colors. It’s important to note that contrary to popular belief, color blindness can appear in females—if a daughter is born to a color-blind father and a heterozygous mother, and happens to be unfortunate enough to receive also receive the recessive allele from her mother · Duchenne muscular dystrophy—caused by absence of the dystrophin protein, coded for on the X-chromosome; victims experience muscle atrophy and loss of coordination · Hemophilia—lack of any of the proteins that participates in blood clotting
X-inactivation One copy of the X-chromosome is inactivated so that males and females have the same number (one!) of effective copies of the X-chromosome
When the X-chromosome is inactivated (by addition of methyl, —CH3), it curls up into a Barr body, hides itself along the inside of the nuclear envelope (because the nuclear envelope has no corners) and cries (jk, it doesn’t actually cry) During meiosis, the Barr body is reactivated so that each ovum has a copy and we don’t end up with people that have 23 chromosomes. Which X-chromosome is inactivated is random, so heterozygous females express 50% of one allele and 50% of the other XIST (X-Inactive Specific Transcript)—RNA made by, attach to, and eventually cover the Barr-body chromosome to inactivate it
Errors in Chromosome Number Nondisjuction—homologues don’t separate in meiosis II, causing one gamete to have two copies of the same chromosome and the other to have zero (also known as aneuploidy—having the wrong number of one or more chromosomes). · Trisomic—having three of one chromosome · Monosomic—missing a chromosome Polyploidy—having more than one set of chromosomes; good for plants, usually not so good for animals; perhaps surprisingly, polyploidy is usually less destructive than aneuploidy Disorders · Down syndrome (Trisomy 21)—three copies of chromosome 21; characterized by: short stature, heart defects, high risk of respiratory infection, mental retardation; susceptible to Alzheimer’s disease, leukemia; chances of producing offspring with Down syndrome increase with age · Klinefelter syndrome (XXY)—male disorder; characterized by small reproductive organs, some female characteristics, sterility · XYY—usually taller; that’s it. · Trisomy X (XXX)—cannot be distinguished from XX females except by karyotype · Turner syndrome (monosomy X, X0)—female disorder; causes sterility Errors in Chromosome Structure Deletion—part of chromosome gets removed (eaten! *chomp* ^^) Duplication—segment gets repeated Inversion—segment gets reversed (for example, ABCDE to ADCBE) Translocation—segment migrates to a different chromosome · Reciprocal—chromosome gives away a fragment and gets one in return · Nonreciprocal—chromosome’s fragment gets stolen! (it doesn’t get one in return) Disorders · Cri du chat (“cry of the cat”)—deletion in chromosome 5; characterized by small head with unusual facial features, mental retardation, cries like a cat mewing · Chronic myelogenous leukemia (CML)—caused by reciprocal translocation between big part of chromosome 22 and small part of chromosome 9 short chromosome 22 (Philadelphia chromosome)
Genomic imprinting—variation in phenotype depending on whether the allele is inherited from the mother or the father Zygote only expresses one of the two copies of an allele so all body cells do too Imprinted identically in all members of a species
Organelle Genes (Extranuclear genes) Located in cytoplasm, mitochondria, and plastids (including chloroplasts) Inherited maternally because all organelles come from the mother Mitochondrial myopathy—caused by mutation in mitochondrial genes; characterized by weakness, intolerance of exercise, muscle atrophy