Habrobracon 499 



Females are always heterozygous for two of the series of mul- 

 tiple alleles and may therefore be xa / xb, xa / xc or some other 

 combination of two of the series. Males are normally haploid 

 and therefore have only one of the alleles, but some males are 

 diploid and are homozygous for any one of the series. Normal 

 males might be either xa or xb. If an xa male mated with the 

 xa I xb female, the offspring would be xa /xb females, xa males, 

 xb males, and oca / xa diploid males. If the male used in the cross 

 was an xb individual, the offspring from the same female would 

 be xa / xb females, xa males, xb males, and xb / xb diploid males. 

 The xa and xb haploid males are indistinguishable phenotypically. 

 Since femaleness is determined only when xa and xb sex alleles 

 (or some other combination) interact, this appears to be a case of 

 complementary genes. Although the stock that was just described 

 contained only the xa and xb genes, another stock might contain 

 only xc and xd. In this stock, females would be xc / xd and hap- 

 loid males xc or xd, but diploid males would be produced by 

 crosses within the stock and would be .re / xc or xd / xd. If the 

 two stocks are crossed together, no diploid males are produced. 

 For example, an xa / xb female mated with an xc male would pro- 

 duce xa / xc and xb /xc females and xa and xb males, but no males 

 with two sex genes or with the gene xc. In Habrobracon all 

 heterozygotes are female whereas all homozygotes and all hap- 

 loids are males. 



A sex-linked gene, fused, has been studied extensively and ap- 

 pears to be linked with the x genes with 10 per cent crossing over 

 (Fig. 143). If an orange-eyed, oo, female heterozygous for fused, 

 Fufu, is mated to a black-eyed, 0, fused haploid male, the diploid 

 males and females will be black-eyed and the haploid males will be 

 orange. In this way, the two chromosomal types of males can be 

 distinguished. If the cross is between members of the same stock, 

 as xb Fu / xafuX xa ju, the diploid offspring will segregate into 



9 nonfused females xh Fu / xa fu 



1 fused female xh fu / xa fu\ 



1 r J 1 r, / J- /Crossovers 



1 nomused male xa tu / xa fu/ 



9 fused males xa fu / xa fu 



and the haploid males will be in the ratio of 1 nonfused (9 xb Fu 

 and IxaFu) to 1 fused {9 xa fu and Ixbfu). If the original 

 cross is xb Fu /xa fu X ^b fu, or xa Fu / xb fu X ^cl fu or xa 



