Chromosomal Basis of Inheritance Biology 112

Locating Genes Along Chromosomes Mendel’s “hereditary factors” were genes, though this wasn’t known during his time Today know that genes are located on chromosomes The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

The chromosome theory of inheritance states: Mendelian inheritance has its physical basis in the behavior of chromosomes The chromosome theory of inheritance states: Mendelian genes have specific loci (positions) on chromosomes Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis can be said to account for Mendel’s laws of segregation and independent assortment

Figure 15.2 The chromosomal basis of Mendel’s laws P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds ( yyrr) Y y  r Y R R r y Meiosis Fertilization R Y y r Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Meiosis LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Metaphase I Y y Y y 1 1 R r r R Anaphase I Y y Y y Figure 15.2 The chromosomal basis of Mendel’s laws R r Metaphase II r R 2 2 Y y Y y y Y Y y Y Y y y Gametes R R r r r r R R 1/4 YR 1/4 yr 1/4 Yr 1/4 yR F2 Generation An F1  F1 cross-fertilization 3 3 9 : 3 : 3 : 1

Drosophila melanogaster Thomas Hunt Morgan Drosophila melanogaster

Morgan’s Experimental Evidence The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors Several characteristics make fruit flies a convenient organism for genetic studies: They breed at a high rate A generation can be bred every two weeks They have only four pairs of chromosomes

Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes

Sex-Linked Genes Sex-Linked genes: Genes located on sex chromosomes. This term is commonly applied to genes on the X chromosome 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 Morgan determined that the white-eyed mutant allele must be located on the X chromosome

EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + +  Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION P + X w X w Generation X  Y + w w Sperm Eggs F1 + + Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? + w w Generation w w + w Sperm Eggs + + + w w F2 w Generation + w w w w + w

Linked genes Each chromosome has hundreds or thousands of genes Genes located on the same chromosome that tend to be inherited together are called linked genes Linked genes tend to be inherited together because they are located near each other on the same chromosome

Linkage Affects Inheritance Morgan did some experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size

However, nonparental phenotypes were also produced Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) He noted that these genes do not assort independently, and reasoned that they were on the same chromosome However, nonparental phenotypes were also produced Genetic recombinationoccured: the production of offspring with combinations of traits differing from either parent

b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or Fig. 15-UN1 b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most offspring or b vg b vg

EXPERIMENT RESULTS Fig. 15-9-4 P Generation (homozygous) b+ b+ vg+ vg+ Wild type (gray body, normal wings) Double mutant (black body, vestigial wings)  b+ b+ vg+ vg+ b b vg vg F1 dihybrid (wild type) Double mutant TESTCROSS  b+ b vg+ vg b b vg vg Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+ Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b vg Figure 15.9 How does linkage between two genes affect inheritance of characters? Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : : RESULTS 965 : 944 : 206 : 185

Recombination of Unlinked Genes: Independent Assortment of Chromosomes Mendel observed that combinations of traits in some offspring differ from either parent Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants A 50% frequency of recombination is observed for any two genes on different chromosomes

Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome That mechanism was the crossing over of homologous chromosomes

Black body, vestigial wings Fig. 15-10 Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromo- somes Replication of chromo- somes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes Figure 15.10 Chromosomal basis for recombination of linked genes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

Black body, vestigial wings Fig. 15-10a Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromo- somes Replication of chromo- somes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ Figure 15.10 Chromosomal basis for recombination of linked genes b vg Meiosis II Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ b vg Eggs Sperm

965 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Fig. 15-10b Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ Figure 15.10 Chromosomal basis for recombination of linked genes b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

Mapping the Distance Between Genes Using Recombination Data Genetic map, an ordered list of the genetic loci along a particular chromosome The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

A linkage map is a genetic map of a chromosome based on recombination frequencies Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency Map units indicate relative distance and order, not precise locations of genes

RESULTS Recombination frequencies 9% 9.5% Chromosome 17% b cn vg Fig. 15-11 RESULTS Recombination frequencies 9% 9.5% Chromosome 17% Figure 15.11 Constructing a linkage map b cn vg

Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial Fig. 15-12 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 48.5 57.5 67.0 104.5 Figure 15.12 A partial genetic (linkage) map of a Drosophila chromosome Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

Genes that are far apart on the same chromosome can have a recombination frequency near 50% Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes Cytogenetic maps indicate the positions of genes with respect to chromosomal features

Sex systems in animals

Figure 15.6 Some chromosomal systems of sex determination 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X or + Sperm Egg 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ Figure 15.6 Some chromosomal systems of sex determination (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex- linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome

Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness Duchenne muscular dystrophy Hemophilia

(a) (b) (c) Sperm Sperm Sperm Eggs Eggs Eggs XNXN  XnY XNXn  XNY Fig. 15-7 XNXN  XnY XNXn  XNY XNXn  XnY Sperm Xn Y Sperm XN Y Sperm Xn Y Eggs XN XNXn XNY Eggs XN XNXN XNY Eggs XN XNXn XNY XN XNXn XNY Xn XnXN XnY Xn XnXn XnY Figure 15.7 The transmission of sex-linked recessive traits (a) (b) (c)

Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness Duchenne muscular dystrophy Hemophilia

X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character DNA Methylation is the mechanism for X-inactivation (methyl groups added to cytosine nucleotide in the DNA molecule)

X chromosomes Allele for orange fur Early embryo: Allele for black fur Fig. 15-8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Active X Black fur Orange fur Figure 15.8 X inactivation and the tortoiseshell cat

Chromosomal Alterations Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders…bad news!

Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Nondisjunction can also occur in mitosis

Meiosis I Meiosis II Gametes (a) Nondisjunction of homologous Fig. 15-13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes Figure 15.13 Meiotic nondisjunction n + 1 n + 1 n – 1 n – 1 n + 1 n – 1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Nondisjunction video

Aneuploidy Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome

Polyploidy Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Triploidy (3n) is three sets of chromosomes Tetraploidy (4n) is four sets of chromosomes Polyploidy is common in plants, but not animals Polyploids are more normal in appearance than aneuploids

Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure: Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses a segment within a chromosome Translocation moves a segment from one chromosome to another

Reciprocal translocation Fig. 15-15 A B C D E F G H A B C E F G H Deletion (a) A B C D E F G H A B C B C D E F G H Duplication (b) A B C D E F G H A D C B E F G H (c) Inversion Figure 15.15 Alterations of chromosome structure A B C D E F G H M N O C D E F G H (d) Reciprocal translocation M N O P Q R A B P Q R

Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy

Down Syndrome (Trisomy 21) Down syndrome is an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 700 children born in the United States The frequency of Down syndrome increases with the age of the mother.

Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid conditions Males Klinefelter syndrome is the result of an extra chromosome in a male, producing sterile XXY individuals (1 in 2000 births?) Extra Y (XYY)…apparently normal males (1 in 1000 births?) Females Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans (1 in 5000 births?) Triple X Syndrome (XXX)…apparently normal females (1 in 1000 births?)

Genomic Imprinting For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits Such variation in phenotype is called genomic imprinting Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production Causes certain genes to be differently expressed in the offspring depending upon whether the alleles were inherited from the ovum or from the sperm cell.

Fragile-X syndrome (CGG) Huntington’s disease (CAG) Triplet Repeats Fragile-X syndrome (CGG) Huntington’s disease (CAG)

Extranuclear Genes Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm Mitochondria, chloroplasts carry small circular DNA molecules Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg