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A cross in drosophila involved the recessive, x linked genes yellow (y), white (w), and cut (ct). a yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. the f1 females were wild type for all three traits while the males expressed the yellow body and white eye traits. the cross was carried to the f2 progeny and only male offspring were evaluated. on the basis of the data shown here, a genetic map was constructed. what are the genotypes of the f1 females and males? how did you decide?

Respuesta :

mathmate
An interesting scenario!

First, recall that x-linked recessive genes are always expressed in males (who have only one x-chromosome).  Females must have homozygous recessive genes to be expressed.

The above fact allows us to construct the genotypes of the parents, as tabulated below for clarity.

Genotype and phenotype of parents
Trait         XX (expressed)          XY (expressed)
yellow      yy (Y)                          Y (N)
white        ww (Y)                        W (N)
cut           CTCT [CTct]* (N)    ct  (Y)

Genotype and phenotype of f1
Trait         XX (expressed)       XY (expressed)
yellow      Yy (N)                      y (Y)
white        Ww (N)                    w (Y)
cut           CTct (N)                   CT (N)

Verify and convince yourself that the above produces the phenotypes as given in the question.

* it is possible that the parent female carries the heterozygous CTct.  However, CTct is possible but very unlikely because ALL offsprings have heterozygous Ctct genes,  which must take only the CT allele from the mother.
marianaegarciaperedo

x-linked genes are genes associated with the sexual X chromosomes. So they are only inherited from the mother/female side. Males express the phenotype defined by the allele received from the mother.

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Available data:

  • Three recessive diallelic genes are involved in this cross
  • These genes are X-linked
  • Gene Y codes for body color  →  Allele Y dominant →  normal

                                                          →   allele y recessive →  yellow

  • Gene W codes for eye color  →    Allele G dominant →  normal

                                                         →   allele g recessive →  white

  • Gene Ct codes for wings shape  →   Allele Ct dominant → normal

                                                               →   allele ct recessive →  cut

  • Female →  yellow-bodied, white-eyed with normal wings
  • Male  →  eyes and body were normal but whose wings were cut

Let us first analyze genotype and phenotypes.

For a better representation, I will stablish symbols representing the alelles

  • Y  Normal body color  → a+
  • y   yellow body color  →  a₋
  • W Normal eye color →  b+
  • w  White eye color  →   b-
  • Ct  Normal wings  →  n+
  • ct   cut wings        →   n-

GENOTYPE                                PHENOTYPE        

Xᵃ⁺Xᵃ⁺  -   Xᵃ⁺Xᵃ⁻  -  Xᵃ⁺Y         Normal body color

Xᵃ⁻Xᵃ⁻  -   Xᵃ⁻Y                          Yellow body color

Xᵇ⁺Xᵇ⁺  -   Xᵇ⁺Xᵇ⁻  -  Xᵇ⁺Y         Normal eye color

Xᵇ⁻Xᵇ⁻  -  Xᵇ⁻Y                           White eye color

Xⁿ⁺Xⁿ⁺  -  Xⁿ⁺Xⁿ⁻  -  Xⁿ⁺Y          Normal wings

Xⁿ⁻Xⁿ⁻   -  Xⁿ⁻Y                          Cut wings                  

Now let us represent the cross

  • Female  → Xᵃ⁻ ᵇ⁻ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁺     or        Xᵃ⁻ ᵇ⁻ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁻  
  • Male  →  Xᵃ⁺ ᵇ⁺ ⁿ⁻ Y

Cross) Male x female

There are two options of crosses according to the female genotype

Parentals) Xᵃ⁻ ᵇ⁻ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁺    x     Xᵃ⁺ ᵇ⁺ ⁿ⁻ Y

                 Xᵃ⁻ ᵇ⁻ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁻    x     Xᵃ⁺ ᵇ⁺ ⁿ⁻ Y

F1)

  • Females  →  Wild type    →   Xᵃ⁺ ᵇ⁺ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁻   →  Heter0zyg0us for all the genes
  • Male →  Yellow body and white eyes  →  Xᵃ⁻ ᵇ⁻ ⁿ⁺ Y  or    Xᵃ⁻ ᵇ⁻ ⁿ⁻ Y

According to the statement of the problem,

⇒ the females in the F1 are wild type for all the traits.

  • Male parent could only provide dominant alleles for body and eye color, and a recessive allele for wings shape

  • Female parent could only provide recessive alleles for body and eye color. The female parental can be either h0m0zyg0us or heter0zyg0us for wings shape, so it can provide either a dominant or a recessive allele for wings shape.

Because F1 females express the wild type phenotype for wings shape, we can assume that they inherited the dominant allele from the mother.

So females must be heter0zyg0us all the traits ⇒ Xᵃ⁺ ᵇ⁺ ⁿ⁺ Xᵃ⁻ ᵇ⁻ ⁿ⁻

⇒ the males in the F1 have yellow body and white eyes.

Male inherited from the mother the recessive alleles for eye color and body color. It could inherit a dominant allele for wings shape, or a recessive one.

So the male genotype can be either   Xᵃ⁻ ᵇ⁻ ⁿ⁺ Y    or   Xᵃ⁻ ᵇ⁻ ⁿ⁻ Y

Considering that the statement only specified the recessive phenotype for body and eye color, we could assume that the male expresses the dominant phenotype for wings shape   ⇒   Xᵃ⁻ ᵇ⁻ ⁿ⁺ Y

F1 Genotypes

  • female   →     Xᵃ⁺ ᵇ⁺ ⁿ⁺  Xᵃ⁻ ᵇ⁻ ⁿ⁻   ⇒ Heter0zyg0us for the three alleles
  • male     →     Xᵃ⁻ ᵇ⁻ ⁿ⁺  Y  ⇒  Recessive phenotyope for eye and body

                                                         color and wild type for wings shape

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